Add "tljh-config unset" option

[GeorgianaElena]

• Add / update documentation
• Add tests

closes #203

I added the option to unset (remove) a config property. @yuvipanda, I'm not sure if this is wanted but I thought it would be nice to be able to remove a config property the same way we're setting it, without editing the config file.

[yuvipanda]

This is very useful! Thank you very much :) <3

We should probably also have a way to add an explicit key-value pair to config as well. Currently we can't set LTIAuthenticator properties, since they look like:

c.LTIAuthenticator.consumers = {
	'key1': 'value1',
	'key2': 'value2'
}

Not sure what the terminal UX for this would look like.

[GeorgianaElena]

@yuvipanda, I'm not sure I understand why the following won't do the job.

tljh-config set c.LTIAuthenticator.consumers.key1 = value1
tljh-config set c.LTIAuthenticator.consumers.key2 = value2

Is it because we assume the authenticator will will only have one-level attributes?

When you say

add an explicit key-value pair to config

do you mean adding a key as it is without parsing it? If this is the case we could use the set command and add a flag that would tell us whether or not to treat the key as a path.

Sorry if I'm far from the truth and you meant something else.