Hotfix

As discussed on twitter I introduced the concept of a Hotfix. I decided to duplicate the code for the moment. I wanted to do it with the least amount of effort but including tests because I don't know if you want it in the migration library or not (but would be awesome).

As we see it Hotfixes are only applied once and only a specific hotfix. That is why this PR provides a HotfixRunner where you can say which version or hotfix name you want to apply (see tests for the usage).