Add :cargo command

[Bedis Nbiba]

Currently we have a lot of commands like :expand :asm that just wraps cargo plugins, I don't think its realistic to support all cargo plugins.

The user currently can use irust scripts for unimplemented commands but that seems to much work.

I propose adding :cargo $args command that just invokes cargo inside irust repl directory and forwards the user argument to it.
I think it should make things a lot better.

One implementation note, parsing user arguments should take in consideration arguments in quotes like :cargo plugin "quoted arg" arg2