Add option "--formatted-output" to make stdout print JSON formatted test results

[johnsusek]

Hello, we are writing a GUI to create alerts, built on top of bitsensor's api.

When testing a rule, the output that comes back isn't structured, so can't be parsed easily by a client.

This change adds a new option --formatted-output for elastalert.test_rule that outputs JSON results.

If this gets merged, a companion PR will get submitted to bitsensor's fork to take advantage of it.

Here is an example output for full run:

{
  "writeback": {
    "elastalert_status": {
      "hits": 11,
      "matches": 11,
      "@timestamp": "2018-09-11 16:55:07.546522+00:00",
      "rule_name": "test321",
      "starttime": "2018-09-10 16:55:03.854197+00:00",
      "endtime": "2018-09-11 16:55:03.854197+00:00",
      "time_taken": 3.6562910079956055
    }
  },
  "hits": 11,
  "terms": {
    "syslog_severity_code": 6,
    "service": "Vpxa",
    "tags": [],
    "syslog_severity": "informational",
    "@timestamp": "2018-09-10T18:16:23.461Z",
    "syslog_facility": "user-level",
    "host": "c5014-cesx121.ms",
    "@version": "1",
    "dept_id": "ms",
    "message": "- - [Originator@68762018-09-10T18:16:23.450Z Vpxa: verbose vpxa[FFC9AAE0] [Originator@6876[VpxaHalCnxHostagent::ProcessUpdate] Applying updates from 11843011 to 11843012 (at 11843011)",
    "syslog_pri": "14",
    "type": "syslog",
    "syslog_facility_code": 1
  },
  "name": "test321",
  "success": true,
  "days": 1
}

Here is an example output for countOnly:

{
  "hits": 11,
  "terms": {
    "syslog_severity_code": 6,
    "service": "Vpxa",
    "tags": [],
    "syslog_severity": "informational",
    "@timestamp": "2018-09-10T18:16:23.461Z",
    "syslog_facility": "user-level",
    "host": "c5014-cesx121.ms",
    "@version": "1",
    "dept_id": "ms",
    "message": "- - [Originator@68762018-09-10T18:16:23.450Z Vpxa: verbose vpxa[FFC9AAE0] [Originator@6876[VpxaHalCnxHostagent::ProcessUpdate] Applying updates from 11843011 to 11843012 (at 11843011)",
    "syslog_pri": "14",
    "type": "syslog",
    "syslog_facility_code": 1
  },
  "name": "test321",
  "success": true,
  "days": 1
}

Here is an example output for schemaOnly:

{
  "name": "test321",
  "success": true
}

[Qmando]

Looks good. Could you add a small bit in the documentation?