Format selected lines

[jeffschwartz]

It would be advantageous to be able to apply formatting to only selected lines.

[jdavisclark]

+1. Super useful when you want to clean up a few lines but aren't at a point where you can commit whitespace changes to everything else.

[ilanbiala]

Waiting on https://github.com/atom/atom/issues/4565 to see what to change, because the code looks right but doesn't work when run in the package. Running it through the developer tools works though, which is weird.