Support and respect underlying tool's `--fix` feature

[dlukeomalley]

Proposal is to respect, where possible, a tool's underlying fixes. Related ask from a user:

I also wouldn't mind a way to apply changes (kind of like python's 2to3 converts files).

[ulziibay]

Honestly https://libcst.readthedocs.io is way better than 2to3 for enabling this feature. See https://github.com/returntocorp/r2c-authentication-checks/pull/4 for previous work!

[ievans]

@ulziibay unfortunately libCST only supports python 3.5+ as input, so it couldn't do 2to3 workflow

[dlukeomalley]

User comment:

• There were some things that would be nice to fix, but would be a pain to go through and manually fix them. Ex: == vs ===. Would be nice to have something like tslint's tslint --fix.

[aryx]

Spatch can also implement easily many autofixes. See https://github.com/facebookarchive/pfff/wiki/Spatch I have a version that can work on many languages that is similar to the recent paper by claire Goues on parser combinators for program transformation.