Show a summary for each command (cargo --list)

[malbarbo]

Running cargo shows a list of commands with a summary for each, but cargo --list does not show a summary.

Probably a summary can just be showed for internal commands.

[alexcrichton]

Seems reasonable to me!

[dwijnand]

Had a look at this and two things:

1. The strings that cargo prints don't match the about of the commands. For example for build, cargo prints "Compile the current project", while the about string is "Compile a local package and all of its dependencies". Is that fine? Which one should --list output?

2. --list supports -v to output the path of the command (TIL!). Given the descriptions we want to add are for built-in commands, which don't have an associated "path", do you think it's fine to mix descriptions with paths, or is that confusing?