add option to view expirations of reservations

I tried to use the copy module but a shallow copy wasn't sufficient to preserve the reserved_groups collection and a deep copy bombed out.  The duplicate call is kind of gross though, and I just realized it happens every invocation rather than only when --show-expirations is provided...

Seems like this pushes things close to wanting to be broken out / refactored but I figured it would be useful for at least as a starting point so here it is.