Use atom.clipboard instead of copy-paste module

Now there's no need for the copy-paste module and its crazy dependencies.
Hopefully this fixes the install problems Windows users were having.

Fixes #223, #210, #186.

Test Plan:

On OS X, reloaded an Atom window. Caused a lint message to appear then verified
that clicking the clippy icon copies the lint message text to my clipboard.
Didn't test anything on Windows, but crossed fingers that this is a fix.