Abstract `config set --add` to `config add`

[danielbachhuber]

config set --add is a deviation from our standard syntax. Typically, set and add are two separate operations (e.g. option set vs. option add, and cache set vs. cache add).

It'd be better to abstract config set --add to config add.

We may also consider a config update, which would add the constant if it didn't exist, or update it if it did.

[danielbachhuber]

@wp-cli/committers Does this seem reasonable to you?

[gitlost]

Seems reasonable to me.

[frankiejarrett]

Agreed.

Using the same CRUD verbs as the option command seems the most straightforward to me.

wp config get
wp config add
wp config update
wp config delete

[schlessera]

Hehe, I think I started out with wp config add & wp config update, but then later changed it.