Make it easy to run HELM on new models

[yifanmai]

Right now users have to fork and change the repo itself. Ideally, users can just pip install the package, define the client outside the package, and it should still work.

Also, we need to add documentation for how to do this.

[percyliang]

Do we want to do this? One consideration is that it'd be nice if things were contributed back to the repo. Perhaps it'd be simpler if things outside the repo were just conf files? What do you think?

[yifanmai]

This was done in #1761 and #1787.

We still need to write documentation, but the API is still in flux, so I would like to pin that down first before adding documentation.