Offlineimap (or other mail fetchers) may query the user for a
password. This patch adds a process filter to the `mu4e-update-mail'
process and checks if the process asks for a password (Currently only
matches "^Remote: Enter password: $"). It then reads the password from
the user.
Signed-off-by: Rüdiger Sonderfeld <ruediger@c-plusplus.de>