notmuch configuration featuring multiple smtp accounts

WARN: this commit WILL break an existing mail sending setup.  you need
to set the variable smtp-accounts and provide login credentials
through ~/.authinfo.gpg.

if you want to continue to send email through a local sendmail change
message-send-mail-function to your needs.

a typical mail configuration looks now like:

(setq user-mail-address "coroa@online.de"
      user-full-name "Jonas Hörsch"
      smtp-accounts
      '(("coroa@online.de" "Jonas Hörsch" "smtp.gmail.com" "coroan" "submission")))
custom.el cleaning (coroa)