Highlight angle braced hyperlinks.

Fixes #104

Follows specification at: http://jgm.github.io/stmd/spec.html#autolinks . Note that that spec does not allow autolinks without braces, even though many implementations do it.

Also fixes autolinks without angle braces which were not being highlighted on top level, because I had to add them to the toplevel cluster.

Does not conflict with HTML tags because only strings with certain protocols are allowed.