Add support for Cosmo theme

[airborn]

There is one quite new Bootswatch theme called Cosmo http://bootswatch.com/cosmo/

[joshmoody]

Instead of trying to keep all the bootswatch themes up to date in this project, what do you think about pulling the latest version from the bootstrap cdn?

linkEl.href = '//netdna.bootstrapcdn.com/bootswatch/latest/' + theme + '/bootstrap.min.css';

Then there is no need to update this repo for new themes or updated versions of bootstrap.