notmuch: Add a 'part' subcommand

A new 'part' subcommand allows the user to extract a single part from
a MIME message. Usage:
  notmuch part --part=<n> <search terms>
The search terms must match only a single message
(e.g. id:foo@bar.com). The part number specified refers to the part
identifiers output by `notmuch show'. The content of the part is
written the stdout with no formatting or identification marks. It is
not JSON formatted.