Detect branch and pull request merges

When a branch gets merged, the github notification looks like this:

```
13:57 <__bot__> [anlutro/botologist] 20 new commits to master by anlutro -
https://github.com/anlutro/botologist/compare/2dac0079ba2e...fb525de79a10
```

When a pull request gets merged:

```
15:30 <__bot__> [anlutro/botologist] Pull request closed by anlutro: tvseries plugin: tell when series
are cancelled - https://github.com/anlutro/botologist/pull/88
15:30 <__bot__> [anlutro/botologist] New commit to master by moopie: tvseries plugin: tell when series
are cancelled - https://github.com/anlutro/botologist/commit/a1bdeeabcd
15:30 <__bot__> [anlutro/botologist] New commit to master by anlutro: Merge pull request #88 from
moopie/patch-1 - https://github.com/anlutro/botologist/commit/b77a305760
```

It could look at the first/last commit, look for a merge-like commit message and report that instead.