Add Command::mut_args to modify all arguments of a Command

[joshtriplett]

This implements the mut_args method proposed in #5050 .

Convenient when you want to modify most or all arguments, without having
to call mut_arg for each one and name them all.

There may be a more efficient implementation that doesn't involve recreating
an entire Vec.

Fixes #3002

[epage]

FYI we had #2966 and then reverted it in #2994 because the use case was limited and handled in other ways while the old implementation of clap led to the design being more complicated.

Since then, we've been tracking this in #3002.