Add client-certificate PEM file support for mirror-url over https

[baylanger]

This option allows to specify on the command-line a PEM file to use as client certificate when connecting to a mirror-url over https.

$ spoa-mirror --client-certfile=/path/ssl/clientcert.pem --mirror-url https://vip_sslclient_yourdomain.com/ ...

This "ninja" solution has been tested with success on first attempt. We're hoping you don't mind merging this, please notify if any changes to make this happen.

Thank you.