Support old-fashioned patches

[paradox460]

The PatchSet and PatchDefinition systems have many advantages to traditional patches, but one shortcoming is they can't work with plain text files. Some mods use plain text to store things like MOTDs, and it would be nice to patch in a reliable way. I've been using interpolation where appropriate, but it's not ideal.

Having the ability to use .patch files, as used by the unix patch command, would be extremely useful. While these are more fragile than the path-based system, they also have the ability to do far more variety of changes

[paradox460]

This would make the changes I'm attempting in #445 absolutely trivial, for example

[itzg]

Yeah, the current patching is just a best effort implementation using the library that was already in use for JSON processing. Generic patch file support makes sense to include as an option.

I'd be happy for a sensible PR for this since I probably won't be able to get to this for quite some time.