Add support for installing the formatter directly through cargo

The rust package manager has a feature where you can [publish binaries to crates.io and install them through cargo](https://doc.rust-lang.org/cargo/reference/publishing.html). Like you could run `cargo install starship --locked` and it would install the Starship shell prompt and add it to the path. It seems a lot more convenient than downloading the binary and manually adding it to the path. You can also clone the source code and install it through cargo that way, but having a command that you can copy-paste if you already have Rust installed would be pretty nice.

The pre-built binaries should still be there as an option, but the current installation process seems a little convoluted is all.