Multiple app support

Right now `up` is centered around a single deployable. This means you have to `cd api` and work with it there, and then `cd ../site` to work with another.

This is not such a bad thing, but it may be nice to have some sort of multi-app awareness down the road. This is more critical of the desktop app than the CLI for now.

Potentially many of the additions to Up will be written as Up apps as well, so that helps make it more relevant. Ideally you can run stuff like `up apps` to get a basic view of where versions are at and so on.

This will also help Up supersede apex(1) eventually, if that's ever a direction worth taking.