Teach _Wikitext_parser_encode_link_target to take a parser parameter (wikitext, 605aba1)

Instead of passing a string object, pass a pointer to a parser struct instead. This is groundwork for teaching _Wikitext_parser_encode_link_target how to handle links when "treat slash as special" is in effect.

Signed-off-by: Greg Hurrell <greg@hurrell.net>