Add support for nested parentheses in links

[Ambrevar]

The fontification of the link stops at the first closing parenthesis, which is wrong when the link contains parenthesis itself.

[foo](bar(baz)qux)

I guess an easy fix would be to match against )\s, with \s being unicode spacing. (Not sure this is a valid Emacs regexps.)

[syohex]

I think it is not easy to fix. Regular expression cannot handles nested pairs. I suppose we can fix this issue by using lambda expression matching instead of regular expression, however this is big change.

[Ambrevar]

That's why I suggested the "easy fix". Don't bother with nested parentheses if URI cannot contain spacing (linebreak included), just match against )\s: the inner parenthesis will be included in the link.