Allow any file extension

This patch set:
- If the ``extensions`` setting is empty, allows any file extension.
- Adds a toggle command to the command palette, which is useful when all file types are enabled.
- Fixes several little niggles that I discovered in the process of the above:
  * ``SublimeWritingStyleListener.handle_event()`` now calls ``mark_words()`` on newly-opened files, so you don't need to dirty the buffer before a style check kicks off.
  * When ``mark_words()`` is given ``search_all=False``, it now does a case-insensitive match on the word lists.

Minor caveat: because the ``extensions`` check is now just a dict lookup of file extension and file name, this breaks anybody counting on the ``endswith()`` check of the existing implementation.

Let me know if you want this broken up or if you want it squashed into one commit.