Adapt checks to fit repo's merge strategy

[zeke]

GitHub PR can be merged in three ways. Let's call them merge, squash, and rebase.

This bot should check the repo's merge button configuration and change its behavior accordingly:

• If merge or rebase, then a semantic PR title is not adequate, as it won't become a commit message. In this case, at least one semantic commit is required in the PR.
• If squash, then a semantic PR title is required, as any semantic commits in the PR will be squashed.

[zeke]

Further info:

https://developer.github.com/v3/pulls/#merge-a-pull-request-merge-button API can be used to explicitly set the merge_method to squash. This would work if the bot was merging the PRs.
Humans could summon the bot to merge by either adding a label (e.g. merge) or with a comment (e.g. /merge). Note that in the case of using a comment, the bot would have to check that the commenter actually has permission to merge the PR.

Also, an edge case for squash commits (via @francisfuzz):

I was reminded of something I learned about how we treat squash merges recently and would like to share it here in the case it hasn’t been considered by the Electron team.

Squashing a pull request with one commit would use that commit’s message, but squashing a pull request with more than one commit will use the pull request’s title for the commit message.

[zeke]

This issue was mostly resolved by #19; the unresolved part is now #17