Custom CodeWorld configurations

A few people have expressed interest in more CodeWorld configurations. There are currently two: the core configuration, and the Haskell configuration.

Ideas for more configurations:
- Include a library for compass-and-ruler constructions a la geogebra created by Fernando Alegre.
- Include an embedded DSL for physics based on https://hackage.haskell.org/package/learn-physics by @walck
- Fixed long-term stable API versions for codeworld-base or codeworld-api, which would be guaranteed to remain fixed for a class, or even for a multi-year sequence.

Configurations would need:
1. Libraries that should be installed and made available.
2. Customized compiler options.
3. Custom error message processing. (And if we start preprocessing source code, custom plugins for that, too.)

There are open questions about the UI:
- How do you create a configuration? By sending a pull request and asking for a new site release?
- How do you select a configuration to use for your project? Currently, each configuration gets a different base URL and hash prefix.