Add support for using documentation siphons in multiple ways

Add support for using documentation siphons in multiple ways

Experiental support for generating  multiple output formats from the
same siphoned data.

Adds a contrived example to generate a plain list of all CLI commands
(the "itemlist" format).

Eventually we can consider moving the tempate procesisng into the
Output class as well as a way to override how the data is traversed
(ordered).

Change-Id: I77629a74a8fa0c7e583993469dc50491f72f13e7
Signed-off-by: Chris Luke <chrisy@flirble.org>