Installation by repo cloning

[roolebo]

It'd be good to provide another way to install the CLI by repo cloning and sourcing a file. This should work better with things like rcm and version-controlled shell config. Right now it's not clear how do I make slack-theme-cli a part of my rcm config.

A subtle bonus of repo cloning is the ability to add a slack-theme update that could be implemented by a simple git pull of the repo, like it's done in oh-my-zsh.