This PR does a few things:
- supports async mode for sending mails
- registers imagemagick as handler for images if it exists
- sets default downloads directory if it exists
- sets a few (more) sane defaults
- supports format=flowed in messages
Currently, in multi-account mode, the mu4e layer guesses which account
to use to reply a message by its "maildir". However, some messages may
be forwarded and may need to use the original email address to reply.
This commit detects whether such an email address is defined in
"mu4e-account-alist", and uses that to do the replying if possible.
