Outputs a log of updated features

[ChrisC]

Generates a json file listing updates made during a run of updated-bcd. Primarily for further use in BCD update automation.

We're initially just capturing the BCD path, browser and updated support statements in the log file, but there's a lot of additional data from the UpdateLog object that we can add as needed.

cc @teoli2003 @foolip

Closes #946

Sample output from a recent run:

[
  {
    "browser": "firefox_android",
    "path": "api.AudioProcessingEvent",
    "statements": [
      {
        "version_added": false
      }
    ]
  },
  {
    "browser": "firefox_android",
    "path": "api.AudioProcessingEvent.inputBuffer",
    "statements": [
      {
        "version_added": false
      }
    ]
  },
  {
    "browser": "firefox_android",
    "path": "api.AudioProcessingEvent.outputBuffer",
    "statements": [
      {
        "version_added": false
      }
    ]
  },
  ...
]

[foolip]

Code changes look good to me, but I'd like @teoli2003 to confirm this will work for the updating workflow.

[teoli2003]

Looks good for me too!