send-email: add ability to send a copy of sent emails to an IMAP folder

send-email: add ability to send a copy of sent emails to an IMAP folder

Some email providers like Apple iCloud Mail do not support sending a copy
of sent emails to the "Sent" folder if SMTP server is used. As a
workaround, various email clients like Thunderbird which rely on SMTP,
use IMAP to send a copy of sent emails to the "Sent" folder. Something
similar can be done if sending emails via `git send-email`, by using
the `git imap-send` command to send a copy of the sent email to an IMAP
folder specified by the user.

Add this functionality to `git send-email` by introducing a new
configuration variable `sendemail.imapfolder` and command line option
`--imap-folder` which specifies the IMAP folder to send a copy of the
sent emails to. If specified, a copy of the sent emails will be sent
by piping the emails to `git imap-send` command, after all emails are
sent via SMTP and the SMTP server has been closed.

Signed-off-by: Aditya Garg <gargaditya08@live.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>