Rewrite of POD::Readme

This is a major rewrite. Basically,
- Rather that subclass a POD parser, it's a simple POD filter
- A separate method runs a POD translation step
- The pod2readme command handles command-line options
- It supports plugins for added functionality, and includes plugins to insert the module version, 
  latest changes and requirements.

Still to do:
- more tests, especially for plugins and file conversion
- convert from Moose to Moo

typo - "which parts are ..." probably?  or "which parts of the documentation are ..."?