Tweaks for the new filetype handling.

This is a follow-up to PR #16.

1. The ftplugin does not have to source ftplugins for HTML. This is
handled by setting `ft=html.twig` when editing foo.html.twig and bar.twig,
and it is wrong when editing foo.js.twig.
2. Do not rely on ftplugins already defining `b:undo_ftplugin`.
3. OMG, did I leave off `let` in the syntax file?!?
4. I do not see any reason for `:unlet! b:did_ftplugin`.
5. Since the ftplugin does not set the `'commentstring'` option, it is not responsible for resetting it in `b:undo_ftplugin`.