In 423bf7b, I tried to add fallback mechanism to ``python3``. But it breaks
the python3 highlighting on python2 environment.
This adds ``'default'`` to highlighting languages; it works like
``'python3'``, but fallbacks if failed Highlighting. And this removes
try-parse step from ``'python3'`` language. Now, it highlights regardless of
runtime environments. Thanks to Yoshiki SHIBUKAWA.