Implement simple tilde expansion

This at least partially implements #1136. This does not support `~someotheruser` notation.

Once I learned more about how backticks worked, it was so dirt-simple that I'm suspicious this is overlooking something big. @StefanKarpinski in particular please review.

I'd really like to get this merged for 0.2. Globbing would be even nicer. Is shell-parse the right place to put that? It seems quite a bit more complicated.