Add support for setting SSL certificate options through the Web interface

[d3adb5]

As far as I've gone, I haven't seen options anywhere on the default Web interface to set the following properties on the ZNC configuration:

SSLCertFile = /path/to/combined/cert.pem
SSLKeyFile = /path/to/private/key.pem
SSLDHParamFile = /path/to/DH/file.pem

As it stands, I can only do so through manually editing the configuration file. I'd like to avoid having to do that, and if the options are there or it can be somehow done through environment variables, do let me know. 😦