Add subcommands for `navi repo` (list, remove)

[HidegonSan]

I think it's useful. (sorry if there are already solution.)

When I tried to arrange my fatted repos, I needed to remove manually.

list shows a repository list already installed to computer.
remove <REPO> removes cheatsheets installed from specific repository.

Thanks!

[welcome]

Thanks for opening your first issue here! In case you're facing a bug, please update navi to the latest version first. Maybe the bug is already solved! :)

[alexis-opolka]

Hi !

A really good suggestion. ^-^

I was implementing the list feature but "remove" might also be a good idea!

When I'll have time to finish the next pre-release, I'll let you know too.