Hello,

I am moving from 3rd party completion frameworks to use
completion-at-point- only. In the process, I discovered that I want to
override message-expand-name, which is listed in
message-completion-alist. This patch adds a new defcustom for the email
header regular expression, so as to allow users (a) to customise the
regular expression, and (b) to use alist functions for manipulating
message-completion-alist, effectively making programmatic customisation
easier.

Cheers,

  --alexander