Create a branch whenever a new major release is made

[job13er]

We want to make performing maintenance builds on previous releases as easy as possible. To facilitate this, we should have pr-bumper automatically create a maintenance branch on the previous major version whenever a major bump is made. For example, during the merge build of a #MAJOR# change that will bump the version of a package from 1.4.5 to 2.0.0, the pr-bumper would create a 1.x branch based on the tag of 1.4.5 just after bumping master to 2.0.0

Care will simply need to be taken by developers to not use a #MAJOR# bump on a maintenance release branch, as it would cause confusion and likely errors if/when it collided with the main release stream that had already moved to the next major version.

This is the feature @sglanzer requested and @EWhite613 started in #37, but that needed a little work and was held back so I could do some refactoring in #36. I didn't want to forget about it, just in case it proves more difficult than I first thought and doesn't make it in with the bitbucket server changes.