RFC: Enable/Disable plugin

I added disable/enable support but I'm ignoring the whitelist because I think it is too complex.  I can see the utility of a blacklist but I'm thinking pf4j should still Load/Resolve blacklisted plugins but not Start them.  The way it is implemented now, disabling a plugin makes it invisible to the plugin manager for the remainer of runtime.

My (unimplemented) proposal would be:

1. change the definition of *disabled* to mean *do not start*, not *do not load*
2. add PluginState.DISABLED
3. eliminate the whitelist