Multi-user authentication

@nix411 You should be able to just set the environment variables PRODIGY_BASIC_AUTH_USER and PRODIGY_BASIC_AUTH_PASS on the command line when you run Prodigy. For example:

export PRODIGY_BASIC_AUTH_USER=username 
export PRODIGY_BASIC_AUTH_PASS=supersecret 
prodigy ner.teach ... # etc.