Main changes since v5:

   - replace use of notmuch-message-message-insinuate. I'm not sure if
     we should leave a stub function, or just break people's .emacs,
     and tell them not to call it anymore.

   - company is autoloaded, and used, if present. This can be disabled
     by setting notmuch-message-use-company to nil

   - notmuch-message-command being nil is now an explicit disabling of
     of completion

   - deduplicate=address is hardcoded

Things I didn't have time to do

       - date timeout
       - make deduplication configurable
       - make TAB invoke company

I'm still not sure about the use of notmuch-message-command as opposed
to a seperate variable. This particular setup will be a smooth
transition for those that have customized the variable.

I'm also not sure if enabling company should turn off non-company
based completion.

A more conservative default would be to set
notmuch-message-use-company to nil.