Run cargo init as last step if a project doesn't already exist

[cyntheticfox]

Not sure how complex it would be to implement, but running cargo init with the package name would be super useful as a final step. If it can also init as a library when passing the -L flag, that'd be even better.

[yusdacra]

This should be fairly easy to implement. We can check for the existence of a Cargo.toml before running cargo init. I'm not sure if cargo init does this, if it does we can just get away by calling cargo init.