Added the possibility to priorize a language for urlify

Fixes: http://www.concrete5.org/index.php?cID=391912

This is useful if languages have different rules for the same character
e.g. German: "ü" => "ue"
Turkish: "ü" => "u"

The language specified will have the priority over all others.

This could go together with Remo's Pull request:
https://github.com/concrete5/concrete5/pull/877

Also see: https://github.com/jbroadway/urlify/pull/6