Don't ignore all '#' URLs when running HTML::Proofer

Explicitly ignore the clipboard button from proofing, which was why the
:href_ignore was there in the first place.  Per
https://github.com/github/choosealicense.com/pull/104/files#r7311111.
Merge pull request #166 from github/proofer-ignore

Don't ignore all '#' URLs when running HTML::Proofer