Allow Moo Translation Session Re-Use

[geoffreywiseman]

There are times when it makes sense to reuse a translation session, although that should be done carefully because there are consequences to that choice.

In any case, Moo doesn't really expose that as an option right now, and it probably should.

[geoffreywiseman]

We could make each instance of Moo have its own session, but I'm a little wary of accidental session re-use, which won't happen if session re-use is something that you have to do explicitly and which isn't the natural consequence of trying to get some work done with Moo and accidentally going down the wrong path.