For discussion: A slightly more flexible skip plugin

I'm submitting this pull request to generate discussion around the plugin mechanism and to have some example code that will ideally help drive out what you're building.  I would love whatever relevant feedback you can provide, and hopefully having actual code will help us talk more clearly.

Generally, though: I like where this is going. 

If you think this might be a feature that you'd like to merge into the paella core, great! I'll make whatever changes you think are necessary to make that happen. Otherwise, I'm happy to continue managing this as a custom plugin.

I will comment in-line with questions about implementation details and design decisions.
The paella.Plugin base class will register the plugin instance in the PluginManager. You don't need to store the plugin instance unless you need to access it from other plugin. (for instance, if you have two plugins in the same package, and one of them disables the other). If you don't need to do this kind of actions, you can simply do:

    new paella.plugins.FlexSkipPlugin();

In fact, the plugins are not really singletons: you can instantiate a plugin as many times you need (but i don't know whether this can be useful)