Being able to format code by piping it through the formatter makes it much easier to integrate with tools like google/vim-codefmt or Chiel92/vim-autoformat.
Trailing commas after * or ** in a function signature are only safe for Python 3.6
code. So now Black checks whether the file was already Python 3.6 to begin
with. If so, trailing commas are used in such cases. Otherwise, they're not.
When * and ** don't appear in a function signature, the trailing comma is
always safe.
Fixes#8
