Implement the simplest possible form of a "scrape" command for h2s/ppl#23

This isn't done yet. It's not been tested thoroughly enough to release, and it
still has a few bits of missing functionality.

- It needs to autogenerate a contact ID based on their display_name
- It needs to not trip over and die if any of the properties it depends on
  aren't present

I'm thinking this might actually be another prime candidate for a Ppl::Adapter
mail-scraping interface.