Add option to always rebase PR's when out of date with target branch

If you have your Github repository setup to require branches to be up to date before merging, that makes it somewhat harder to use dependabot because you have to ask dependabot to rebase the PR before it is able to be merged.

It would be great if there was a setting that would have dependabot keep the PRs up to date with the master branch so that PRs are always ready to be merged. It might make sense to only use this option in conjunction with a future https://github.com/dependabot/feedback/issues/185 because if you have 5 open PRs and merge one of them, that's 4 PRs that will end up needing to be rebased.

The issue does not name implementation files, tests, or entry points. Start by tracing how Dependabot creates and updates pull requests, then review the related feedback issue about rebasing multiple open PRs. Done means a documented setting reliably keeps eligible PRs up to date with the target branch without creating unnecessary rebases.

Written by the indexing model from the issue text.