Allow to mutate multiple files with one mutation

[NikitaKozlov]

It might be very useful to change multiple files with a single mutation, for example change source files in different modules. Ideally I would love to write a scenario with something like this:

apply-abi-change-to = ["filePath1", "filePath2"]

Please, let me know if you are interested in the PR for this change. I tried it locally and I believe it is a fairly easy change.

[oehme]

Makes perfect sense, a PR is welcome :)

[oehme]

Fixed by #98