A hook for alot that adds format="flowed" to the Content-type header

hooks.py

# This hook needs some changes to the alot code
#
# Currently, only commands initiated from the command line have hooks.
# However, in order to add this hook, we need all commands to have hooks,
# also those created by other commands.
#
# When replying to a message in a thread, the thread.reply command
# is triggered, but it in turn triggers compose which in turn triggers
# edit. These two last commands do not have any hooks attached to them.
#
# The following branch and pull request fixes this:
# - https://github.com/fnurl/alot/tree/hook-always
# - https://github.com/pazz/alot/pull/967

def pre_envelope_edit(ui=None, dbm=None, cmd=None):
    envelope = cmd.envelope
    if envelope is not None:
        content_type = envelope.get_all("Content-type")
        if content_type == []:
            envelope['Content-type'] = u'text/plain; charset="utf-8"; format="flowed"'
        elif 'text/plain' in content_type:
            if 'format="flowed"' not in content_type:
                envelope.add('Content type', 'format="flowed"')