Add support for email notifications

SUMMARY

This commit adds support for sending email notifications using the
same behavioral feature set as Microsoft Teams notifications:

- configurable retries and retry delay
- configurable rate limit
- a separate notification per event
  - e.g., request received, username disabled, sessions terminated

The following settings are configurable:

- destination email address(es)
- sender address
- SMTP server IP/FQDN
- SMTP server port
- client identity
  - hostname provided to the SMTP server
  - as noted elsewhere, some SMTP servers will perform forward/reverse
    DNS lookups to confirm that this provided value matches what DNS
    has recorded for the sending IP Address

Some known weaknesses/lack of support:

- hard-coded email template
  - as of this commit, the template using Textile formatting is
    enabled
  - future efforts are intended to move hard-coded templates out of
    source code constants to on-disk files for easier customization
    by the sysadmin(s) supporting this application

- unauthenticated connections to target SMTP used
  - the initial use-case is a local relay which restricts connections
    based on source IP Address
  - future efforts are intended to provide support for authentication

Behavior worth noting:

- automatic client identity value generated from available DNS
  information if not specified by the sysadmin
  - the `Showmax/go-fqdn` package is used to retrieve the FQDN of the
    sending system, with a fallback to
- email address verification applied using the provided W3C regex
  - using a guide from golangcode.com

CHANGES

The following items note (non-exhaustive) changes since the last
point release. Many of these changes will see further refactoring
as part of GH-22.

- Duplicate client/alert request headers so that in-place formatting
  modifications performed on values in the slice do not spill over to
  the values used for email notifications (where a different
  formatting is used)

- Replace placeholder "getter" methods with full-featured equivalents
  to existing MS Teams methods

- Add new "getter" methods to cover new functionality

- Add flags, environment variables to cover new functionality

- Rename existing generically named functions used for Teams
  notification handling to explicitly mention relation to Teams
  notifications
  - this was needed to make room for email-specific notification
    functionality

- Rework some existing Teams-specific functions to allow use with
  Teams and email notifications (DRY)

- Add email-specific functions, methods, types to mirror existing
  Microsoft Teams notifications support

- Replace placeholder email notification template with a "default",
  mostly text-only friendly format and a Textile formatted template to
  be used with older Redmine systems (prior to Markdown support being
  introduced)

- `MessageTrailer` func updated to take a template string for
  generation of a format-specific branding string
  - e.g., Textile-formatted repo link or Markdown-formatted repo link

- Update config Stringer interface
  - it is growing quite large at this point, might be time to
    rethink how it is handled

- TOML configuration file
  - extended with new settings
  - rename some settings in an effort to better communicate intent

REFERENCES

- refs GH-3

- https://golangcode.com/validate-an-email-address/
  - https://www.w3.org/TR/2016/REC-html51-20161101/sec-forms.html
- https://github.com/Showmax/go-fqdn

NOTICE.txt

@@ -100,6 +100,7 @@ SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 
 
 https://golangcode.com/get-the-request-ip-addr/
+https://golangcode.com/validate-an-email-address/
 https://github.com/eddturtle/golangcode-site/blob/master/LICENSE
 
 MIT License
@@ -177,3 +178,22 @@ AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 SOFTWARE.
+
+
+
+
+https://github.com/Showmax/go-fqdn/blob/master/LICENSE
+
+Copyright since 2015 Showmax s.r.o.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.

README.md

@@ -107,7 +107,9 @@ See also:
   - levels, format and output (see [configuration settings
     doc](docs/configure.md))
 
-- Microsoft Teams notifications
+- Optional notifications
+  - Microsoft Teams
+  - Email
   - generated for multiple events
     - alert received
     - disabled user
@@ -153,7 +155,6 @@ Known issues:
 
 - Documentation
   - The docs are beginning to take overall shape, but still need a lot of work
-- Email notifications are not currently supported (see GH-3)
 - Payloads are accepted from any IP Address
   - the expectation is that host-level firewall rules will be used to protect
     against this until a feature can be added to filter access (see GH-18)