md.ml: Url of string --> Url of href * string (where href = string)
md_parser.ml: parsing of markdown links.
From: http://daringfireball.net/projects/markdown/syntax#link
> This is [an example](http://example.com/ "Title") inline link.
> [This link](http://example.net/) has no title attribute.
md.ml: Url now has 3 parameters instead of 2, the new one is the "title" (as in the "title" attribute of an "a" tag).