Detect code review category based on GitHub url

[alanhamlett]

trafficstars

When the url matches regex github.com/[^/]+/[^/]+/pull/\d+/files let's add heartbeat json payload attribute:

{
  …
  "category": "code reviewing",
  …
}

[LaoshuBaby]

Oh looks my #249 are similar to this issue^

By the way, if wakatime allow customed regex, I guess I can just add readthedocs.io for once, and don't need to add it for different project.

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Regex feature was first requested in #25