POC dynamic ac-source selection for web-mode

Since this is a proof-of-concept, be wary that it may bite you, and has
not been thoroughly tested.

Note that I hacked my copy of emmet-mode a bit, to verify that adding
the pre-auto-complete hook to web-mode let me work around emmet-mode's
assumption that if we're not in a css-mode variant, we must want HTML mode.

I think the change to emmet-mode is a good idea in its own right, so
I'll probably send a merge request to the Emmet maintainer.

Also note that the php-auto-yasnippet ac-source isn't working correctly
in web-mode, and I don't know why not. That's another good example of
an ac-source that may be hard to get working with web-mode.