add support for functions in web-mode-engines-alist

Currently, the user must pass a regexp to check against the buffer name
for setting the web-mode engine. However, sometimes it's convenient to
use a function instead. Add support for using a function.

Example usage:

    (defun my-current-buffer-django-p ()
      (save-excursion
        (search-forward-regexp "{% base\\|{% if\\|{% include\\|{% block"
                               nil
                               t)))

    (setq web-mode-engines-alist
          '(("django". "\\.djhtml")
            ("django" . my-current-buffer-django-p)))