Add draft status to signals

[blang]

Adding the draft status to signals allows to ignore pull requests for
auto-merge if they are in draft status.

Example:

merge:
    ignore:
        draft: true

[palantirtech]

Thanks for your interest in palantir/bulldozer, @blang! Before we can accept your pull request, you need to sign our contributor license agreement - just visit https://cla.palantir.com/ and follow the instructions. Once you sign, I'll automatically update this pull request.

[bluekeyes]

Thanks for adding this! As a signal, this implementation looks good, but I don't think a signal/option is necessary. My understanding is that GitHub does not allow merging draft pull requests, so it should be safe for Bulldozer to always ignore draft PRs when processing merges.

[blang]

Currently only tested this with github enterprise 3.3.1 which actually resulted in the PR to be merged despite being in draft state. I was thinking the same but definitely verified this behavior.