Support `diff` option in `runme fmt`

[mxsdev]

Would be nice to have the output of runme fmt as a diff/patch file, especially with project support.

[sourishkrout]

like e.g. runme fmt --patch or as default output @mxsdev?

[mxsdev]

@sourishkrout

Yeah, I'm imagining it as a flag between patch and "normal" outout.

I personally also think it makes more sense as a default output format, since it is more useful for e.g. reviewing changes before actually making them.