Provide a commandline tljh command for setting config

[yuvipanda]

Editing YAML files on the terminal is not the easiest thing for new people. Indents get screwed up often, and lots of people are not used to nano / vim.

We should offer an easy way to interact with this YAML file from the commandline. Inspiration for this comes from the git config command, which lets you edit your .gitconfig files easily, without having to
manually edit the files. This also helps with validation a lot.

Instead of asking users to add:

user_environment:
   default_app: jupyterlab

to the yaml file, they can instead run:

tljh config set user_environment.default_app jupyterlab

This command would:

1. Load the current config.yaml file
2. Set the requested key to requested value
3. Validate the new config
4. Save config
5. Reload JupyterHub to let config take effect.

For lists, you can do:

tljh config add users.admin yuvipanda

or

tljh config set-list users.admin yuvipanda test1 test2

(using set-list instead of set so people don't accidentally clean out their lists and replace them with one item)

View items with:

tljh config view users.admin

You can reset items with:

tljh config reset users.admin