Support adding classes to the server classpath

[mcmdev]

I'd like to see a way to add custom classes to the server classpath.

My use case
I'm attempting to add a custom Log4J appender to the server.

Alternatives I have considered

• Adding the appender using a plugin. This is what I'm currently doing. The problem is that log messages from server startup aren't used in my custom logger
• Forking Paperclip and building a custom Paper jar, which requires too much maintenance for a simple change like this.
• Use app classloader instead of platform classloader as parent #66 would allow for adding custom classes by starting Paperclip with a classpath argument and by class, but would expose the classloader to additional junk.

Implementation idea
I'd like to propose an option to add a list of custom jar files that are added to the classpath urls. Since other options are currently implemented using system properties, adding a system property for this seems reasonable.