Add support for custom file extensions

[NullVoxPopuli]

No description provided.

[rwjblue]

I'm a bit worried about this being a breaking change. Landing this would mean existing codemods would now be running against non-js files (and would almost certainly error when ran).

I wonder if we can have the host codemod-cli project specifically opt-in to the extensions it supports. Either by having them pass that info in when calling bin-support.js (and we could even default to ts,js) or by specifying something in their package.json that we can read, or something smarter than both of those... 🤔

What do you think?

[NullVoxPopuli]

Sounds good, check my update ;)

[rwjblue]

I just added a test and some README changes to explain how to use custom file types.

Should be good to go once CI is happy...

[rwjblue]

I've updated this to instead add an extra argument to the end (instead of being "inside" the paths or globs thing), but I'm going to do a follow up PR to migrate this to use an object (all these ordered arguments are frustrating).

I want to do that separately though so that I can ensure that existing support still works, and new projects get generated with the POJO syntax.