This PR adds the capability to renovate more than one base branch at a time. For instance, a project may have their released `3.x` version on `master`, while an upcoming `4.x` is being prepared using branch `next`. `4.x` might have a quite different set of dependencies (e.g. some removed or some added) so it's not appropriate to only target `master` and keep rebasing, as it will get messy. Instead, it's necessary to target both `master` and `next` to keep both updated.

Closes #1279