Add a mode for pyi files

[JelleZijlstra]

This is a feature request; we talked about this in person briefly yesterday.

Here's what it could look like:

• Black automatically formats *.pyi files in a package in addition to *.py files
• Stub files mostly follow the same convention with a few changes, as follows:
  • If the body of a function or class is just ..., don't put it on a separate line, but just put it after the :
  • Don't put blank lines between methods or functions, or between classes with an empty body
  • Only put one blank line between other classes.
  • Potentially allow longer lines. Typeshed's coding conventions currently allow infinite-length lines, but I'm not sure we should keep doing that.

See https://github.com/python/typeshed/blob/master/CONTRIBUTING.md#conventions for reference.

[JelleZijlstra]

I started working on this.