IAT! doesn't work with GMail's new compose experience

I wouldn't expect it to because it doesn't use a `textarea` anymore.
It seems to use a `body` element like so:

``` html

```

I haven't done enough investigation to see how easy or hard it would
be to special-case IAT! to make editing the content here possible, but
I did notice that it seems to represent the text as HTML DOM
elements... which isn't so promising.

Anyway, I suspect editing content in GMail is a primary activity for
IAT! users so it'd be nice if we can find a way to make it work with
the new compose experience.