Added optional argument to add Mastodon account

The new command \mastodon allows users to add their Mastodon handle with a link to the profile on the correct instance to the title page. Usage is \mastodon{user}{instance}, i.e. \mastodon{nightdice}{chaos.social} Intended to be used alongside \author etc.

May need some adjustments in order to fix unfug logo position on title page.