allow response body from operation command to be piped into another

[gdereese]

perhaps we:

• append special token strings to the start of all output written by operation command before giving to vorpal.log()
• add vorpal.pipe() handler that
  • strips special tokens before passing on
• add code to start of operation command action validator that
  • strips out args for those strings starting with special tokens that aren't response body
  • find body parameter and sets from previous command response body (if present)

However, I'm not sure if I want to completely hijack how output is piped between commands to do this. Perhaps someone is mixing this vorpal extension with another; this would stunt the flexibility of the combination of extensions.