CLI simplification

This PR simplifies the command line interface for gvp.

Main points are:

* It deprecates the `in` command so simply calling `source gvp` defaults to setting up environment variables, it's the main use case, so it makes sense to make the command briefer.
* Eliminates the `out` command, since if you need to change your `GOPATH` the aim will be to work on the context of a new project, you can simply run `source gvp` to set up the env for that project.
* Eliminates the `source gvp in <command>` function that's used to run a single command in the context of the local Go environment, my experience is that you very rarely will only run one command, so `source gvp` also takes care of this use case.

I'd like confirmation from users that these simplifications won't mess up their workflow, anybody has any thoughts?