Add --import option to iadd

[Dmitriy Morozov <dmitriy@mrzv.org>]

It should be possible to import an email message (either from a file or stdin)
by passing --import option to iadd. If no ID is given, treat the email as a new
bug (i.e. just generate a random id); if an ID is given, append the message to
that Maildir. Need to decide what to do if the message does not fit into the
hierarchy in the latter case.