Make CSS support more robust and flexible

By introducing a few new variables, we make it easier for users to
control whether emmet-mode will generate HTML or CSS.

The new variables are particularly helpful for using emmet-mode in
[web-mode](https://github.com/fxbois/web-mode), where both CSS and HTML
may be present in the same buffer, so major-mode detection is not a
reliable means of deciding whether to generate CSS or HTML.

This should fix issue #14, as well as make it easier for users to handle similar cases themselves.