Make it easier to switch between hosts (and accounts)

Currently, if you want to submit to `itu.kattis.com` instead of `open.kattis.com`, you need to manually edit the `.kattisrc`.

I suggest adding an optional `profiles` section to `kitty.yml` where you can define multiple `.kattisrc` files and names for them (or just avoid configuration and have multiple files like `.kattisrc`, `.kattisrc.itu`, etc). Kitty would then be extended with a new `use` command, so you can easily switch between accounts like `kitty use itu`. This also allows users to use multple accounts on the same host.

This would require some file in which kitty can save state.