Added support for matching parentheses in link URLs

[nicked]

Markdown should support inline links that contain matching pairs of parentheses, eg [my link](http://example.com/link_(with_parens)). These are quite common on Wikipedia for example.

The current implementation is not ideal because of the limitations of Reader, maybe it would be better to add support in there for escaped or balanced pairs of characters?

Also don't like the .filter.count but it seems Substring doesn't support count(where:)