allow user to provide self-signed certificate

[kkoyung]

I added an option -c path to mw to allow user to provide the path to a self-signed TLS certificate.

Once the user adds this option, the mw script will:

1. Overwrite $sslcert with the provided path.
2. Add the option -k to curl to skip verifying the certificate.

It would be useful for users to setup mutt to connect imap/smtp servers with self-signed certificate, like Proton Mail Bridge.