Add support for SSL

I came across some use cases where strict compliance rules enforce that all connections to the db's need to use SSL. It would be nice if gh-ost supported that option.
It looks like go-mysql-driver already has support (https://github.com/go-sql-driver/mysql/pull/89)

Also for reference: https://dev.mysql.com/doc/refman/5.7/en/using-encrypted-connections.html