[git-multimail] smtplib, check certificate

Hi,

It seems that smtplib doesn't check if a certificate is valid (signed by
a trusted CA).

For my personal usage, I patched the starttls code in git-multimail:
only for starttls with smtplib.

This patch is inspired from

https://github.com/graingert/secure-smtplib/blob/master/src/secure_smtplib/__init__.py

It could be easy to add support cert check in for smtps (see
secure_smtplib).

This patch was tested only on git-multimail (v1.2)

It introduces two new options:
  - multimailhook.smtpcheckcert (default false)
  - multimailhook.smtpcacerts (default
                               /etc/ssl/certs/ca-certificates.crt)

Best regards,
Simon P.