Change "git xl install [--local]" to "git xl enable [--global]"

[Felix Zumstein]

I know that the CLI has been inspired by how git lfs works, but I find it confusing to use "installing" for the process of running the graphical installer and again for enabling the extension on a repo or global basis.

Also, I'd expect the default to be local, like you also have to provide a --global flag with things like git config --global user.name.

[Frank Lesniak]

I had the same thought: I primarily work with repos that don't use Excel, so I would not want global enablement to be the default.