Implement context subcommand

[shonfeder]

Closes #41

Implements the context subcommand for managing different development contexts. This includes the following nested subcommands:

• list: list all contexts (with an indication of which one the user is currently in
• new foo: create a new context named foo
• switch foo: switch to context foo

See the integration tests in tests/usage.md for more details.

This also adds a new integration test suite tests/errors.md for recording and testing correct response to user errors.