* helm-source.el: Enable highlighting in setup methods (#145).
Use the default sort fn from the var instead of hardcoding it.
Move common code (sort and highlight to setup method of helm-source
to avoid duplication of code.

* helm.el: Add highlight function for fuzzy.
(helm-default-fuzzy-matching-highlight-fn): New user var.
(helm-fuzzy-default-highlight-match): The fuzzy highlight fn.