Add option for Mastodon

Since Mastodon seems to gain some momentum, it would be nice to be able to add a Mastondon url and icon.

More info: https://github.com/tootsuite/documentation/blob/master/Using-Mastodon/FAQ.md
Logo here: https://github.com/tootsuite/mastodon/blob/master/app/assets/images/logo.svg