Highlight angle braced hyperlinks.

Follow: http://jgm.github.io/stmd/spec.html#autolinks

Does not conflict with HTML tags because only strings with certain protocols are allowed.

Removed hyperlinks without braces: they were not working already, and not in stdm because too hard to parse correctly: http://talk.commonmark.net/t/autolinking-is-not-automatic/73/7?u=cirosantilli