Passing closures to `add_command`?

[MarkLagodych]

Currently, only functions can be passed to add_command. I think it would be more useful if closures were accepted too. Could you please implement this functionality?

[wduquette]

Hi, Mark!

I implemented it using functions because I could figure out how to do it with functions without running afoul of the borrow checker. (I'm still at best an intermediate Rust programmer.) You're welcome to fork Molt and give it a try, though.