Add tljh-config command

[yuvipanda]

We do not want users to hand-edit YAML files. This has been a
major source of bugs and confusion for users in z2jh. Doing so
in a terminal text editor makes it even worse.

This lets users type commands directly to modify config.yaml file
rather than edit files directly. This makes it a lot less error
prone and user friendly.

Advanced users can still edit config.yaml manually.

Still needs:

• 'add' and 'remove' subcommands for dealing with lists
• property path support for 'show'
• A topic guide on how tljh-config works
• A How-to guide on adding / setting admin users with tljh-config
• A how-to guide for DummyAuthenticator - enabling it, setting password, restart

Fixes #38