Attachment Support for JSON & XML Notifications

[caronc]

Description:

Related issue (if applicable): #416

JSON and XML notifications now support attachments.

• XML Schema Verification cleaned up (introducing variation v1.1 which supports attachment) as:

   <Attachments encoding="base64">
      <Attachment filename="apprise-test.gif" mimetype="image/gif">base64 encoded string</Attachment>
      <Attachment filename="apprise-test2.png" mimetype="image/png">base64 encoded string</Attachment>
   </Attachments>

• JSON formatting just introduces a new attribute of type list to the already existing payload message:

  {
   <previous entries>
  "attachments": [
    {
      "filename": "apprise-test.gif",
      "mimetype": "image/gif",
      "base64": "base64 encoded string"
    },
    {
      "filename": "apprise-test2.png",
      "mimetype": "image/png",
      "base64": "base64 encoded string"
    }
  ]

Checklist

• The code change is tested and works locally.
• There is no commented out code in this PR.
• No lint errors (use flake8)
• 100% test coverage

[codecov-commenter]

Codecov Report

Merging #426 (6ec8062) into master (836b729) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #426   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           95        95           
  Lines        12083     12118   +35     
  Branches      2034      2040    +6     
=========================================
+ Hits         12083     12118   +35     

Impacted Files	Coverage Δ
apprise/plugins/NotifyJSON.py	100.00% <100.00%> (ø)
apprise/plugins/NotifyXML.py	100.00% <100.00%> (ø)

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