Config option for disabled SSL protocols (#720)

In the global config section:
SSLDisabledProtocol = <value1>
SSLDisabledProtocol = <value2>
...

Available values are: None, SSLv2, SSLv3, TLSv1
How would you prefer to have it? What's the ZNC convention for flags? Right now it's a direct mapping to Csock::DisableSSLProtocols(u_int).