Add stream, add rl/readln/rls/readlns, add example of mutually-recursive commands

[booniepepper]

• Add stream -- Currently behavior is only defined when it is the initial command
  • This closes out Add a way to opt-out of read-lines in pipes #36
• Add rl/readln/rls/readlns
  • Prefer these over read-line. They have a symmetry with pls/printlns and friends so I think read-line should go away in a next major version.
• Add an example of mutually-recursive commands