Adds function to allow filtering pull requests.

* Example usage of the functionality

(defun filter-wip (l)
  (cl-remove-if (lambda (x) (string-match-p "WIP"
                       (oref x :title)))
                l))

     (setq magit-gh-pulls-maybe-filter-pulls 'filter-wip)
Merge pull request #19 from kidd/filter-pulls

Adds function to allow filtering pull requests.