Custom libraries support

So I am working on a plugin, and notice that a few libraries are added upon starting the server. Since it is obvious that you cannot contain **everything** in your own code, I think it would be a nice feature if you had the choice of putting some maven packages in your plugin.yml or some other file. The server would then download those packages, put them into the libs folder, and add the to the classpath or whatever it does with the other libraries.

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