Rename mgnl setup to mgnl customize-local-config

mgnl setup is confusing to new users as it looks like a necessary step after getting the CLI.

We (czimmermann fgrilli rkowalski) decided to rename it to `customize-local-config`

Its a command for a very special use case, so shorter versions, like "customize" weren't a good fit.

Bonus points for supporting both `customize-local-config` and `customise-local-config`, but just one of them visible in the help