arg: make code more idomatic
arg: don't name unused variables
arg: move to staging area
arg: add more comments to echo example
arg: use playpen instead of showing output
arg: advocate pattern matching for simple arguments
match_args.rs: various improvements

Explain commands in help message and clarify the one-argument case.
arg: hardcode some example output for possible arguments
arg: fix `echo -n`

We need to flush if we don't print a newline, else the echoed output
will be eaten by the prompt string (at least on Mac).