Add support for Firefox

[fregante]

Waiting for

• [x] #73
• [x] AMO's first review.
• [x] API keys setup on Travis

Related: #70 #54

[fregante]

Version 2.1.0 was accepted on AMO so the links added here already work: https://addons.mozilla.org/en-US/firefox/addon/hide-files-on-github-/

Travis should also be ready for the submission, we'll find out on the next release.