Support Creating PRs to PRs to Automatically Fix Issues

Hi there! We've been using dangerjs for a little while and it has really helped automate our code review process. It's an awesome tool that I think every serious project should use.

We'd like to take it further, by automatically opening PRs to fix some of the issues that danger detects. As a simple example, danger could see that there are eslint errors, open a new PR to that branch which ran `eslint --fix` and, if that new PR has no issues, _maybe_ even auto merge it! There all kinds of crazy things we could do, like automatically generating some basic test suites if coverage is too low (with a separate danger warning telling you to fill them in), automatically bumping package.json verisons, and generally just being an entry level developer.

All of this is really just predicated on having a clear API to create a new PR to the current one and committing whatever changes danger has made. Maybe it's already a thing, but I couldn't find it in the danger docs - and if it's not, the github API for creating PRs seems relatively straightforward.

No files or tests are named. Start by reading the DangerJS documentation and GitHub's pull-request API documentation; done means defining and implementing a clear API for creating a PR against the current PR and committing generated changes.

Written by the indexing model from the issue text.