emacs: address: allow internal completion on an individual basis

This commit makes two changes. The first allows the user to override
an external completion method with the internal notmuch address based
completion for an individual buffer.

Secondly, if the user has company-mode enabled then it sets up company
mode (based on internal completion) but disables the automatic timeout
completion -- the user can still activate it in when desired with
standard company commands such as company-complete.