Support for indent-region

So, this seems pretty solid - it doesn't break the existing tests, except for the indentation-specific ones, which I've removed.

I added a customizable variable, `emmet-indent-after-insert`, which turns this behavior on and off, and defaulted it to on.

The `emmet-indentation` variable now sets the number of spaces inserted in `emmet-transform`.  I figured it was safe to rely on its existence, since it's set up in a customized var in mode-def.