Add Firefox support

[jvillalobos]

This adds the necessary clause in the manifest for Firefox support (this will trigger a warning in Chrome, unfortunately). So, ideally, the build process would add/remove this clause depending on the target, and name the file with the .xpi extension for Firefox. In its current form, the extension works correctly in Firefox 45 and above (probably with earlier versions also).

Let me know if you need help getting this add-on listed on the Mozilla Add-ons repo. I'll be happy to assist you.

[fureigh]

Thanks very much for this, @jvillalobos! Looking forward to incorporating. Sounds like it's time for me to write a build script. :)