Ignore PRs based on metadata

[aergonaut]

Not all PRs need code review and it's useful to be able to ignore certain PRs. For now, we could ignore PRs based on a few characteristics:

• ~~labels present on the PR~~ Done in #298
• the target branch
• the head branch

In the config YAML format, this could be like:

ignore:
  labels:
    - wip
    - just for fun
  head:
    - regex|pattern
  target:
    - (more)?patterns?
  status: failed

The status key chooses what kind of status Cody should apply to PRs it ignores. By default it should be false which would mean Cody would apply no status. It could also be pending, failed, or success corresponding to the three possible GitHub status states.