WebDAV Client Certificate Support

Huge props on the WebDAV functionality (and surpassed BitKinex by a mile). I would like to make a request to add support for WebDAV client certificate support for those of us needing to use 2-factor authentication.

Thanks for your feedback and suggestion.

This request is being tracked already:
https://winscp.net/tracker/921

Btw, would you provide me home instructions for setting this up on the server-side, so I can test it? Or maybe a test account on your server?