Allow predicates for matching in browse-url-handlers.

Allow predicates for matching in browse-url-handlers.

* lisp/net/browse-url.el (browse-url-handlers): Allow predicates for
matching in browse-url-handlers.  Adapt docs and customize type.
(browse-url-select-handler): Support predicates in addition to
regexes.
(browse-url--non-html-file-url-p): New defun.
(browse-url-default-handlers): Use above predicate entry instead of
two entries.