[AUCTeX] Extend the set of math-entering environments

[Akim Demaille]

Hi people,

I'm using packages that provide commands that enter into math mode
(prooftree from Paul Taylor).  I couldn't find where to teach AUCTeX
that within the prooftree environment I'm in math mode, and therefore
`-a inserts \alpha{} instead of \alpha, $ are accepted etc.

Thanks :)