Make `pew in venv` start a subshell

Currently `pew in myvenv` is an invalid command. It makes sense for
it to run a sensible default command in the virtual env, and a
sensible default command would be a subshell. This proxies that
currently invalid command to be the same as `pew workon myvenv`.

This is intentionally minimal to start a discussion. In pew2
(sorry for that terrible name) I liked the idea of having `in`
do all of the subcommands, and deprecating the `workon` command.
That's an approach that I think is worth considering, and that
I'm certainly in favor of.

If this is acceptable, we may wish to do a further refactor to
combine the `inve` and `subshell` functions. I'll do more of that
if we agree that having `pew in` be more polymorphic is a good
idea, and think that we'd like to mirror that in the `inve`
function.