feature(yapf): allow range_format

[meijieru]

Enable range formatting for yapf & black

[jose-elias-alvarez]

Are you sure this works? I think this will make range formatting format the entire document, since the arguments aren't getting modified. I recommend taking a look at the implementation for Prettier and see if we can do something similar for these formatters.

[meijieru]

I hacked it and use it for several days.
Just find that they always format the entire file.
Sorry for that, and I will look at it later this week.

[meijieru]

I fix the implementation for yapf and update the documentation.

There are some limitations though:

1. It is line-based.
2. Even range is given, it may change the out-of-range content like whitespace, which comes from the plugin itself.

[meijieru]

For black we should wait for psf/black#830

[jose-elias-alvarez]

LGTM, thanks!