Allow to run custom scripts during updates

[tvdeyen]

We ❤️ Depfu at Blish. We recently started to push our private gem on a private package repository (GitHub packages) and now get Blish updates in our client projects. THIS FEATURE SIMPLY ROCKS!

In order to make updates even more magical, it would be nice if we could define a script (rake task, rails generator, shell, etc) that runs on every Depfu update PR. That would make it possible to run rails schema or data migrations that keep the client project up to date even for larger changes.

We could add something to our CI, but it would be much easier to scope to just dependency updates this way.