Allow to select cargo command

[ayrat555]

Problem:

Currently, cargo.el hardcodes commands that it can execute. It's not possible to execute a command that cargo.el is unaware of.

Solution:

cargo provides cargo --list command which lists all available commands. It can be used to select a command using completing-read.

This approach is used in mix.el - https://github.com/ayrat555/mix.el

[ayrat555]

I implemented it in https://github.com/ayrat555/cargo-mode