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.
We are thinking about adding a mechanism to include dependencies, in order to allow to use other javascript libraries in your plugin (if it's compatible with jQuery). We also are planning to add a "custom-plugins" and a "custom-themes" folders to place plugins and skins outside the Paella core.

We probably add the "custom-plugins" and "custom-themes" folder for the current release of Paella 4.