For discussion: A timecode 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.
This convention is really, really confusing. Maybe it's a common pattern in the javascript world that I've not seen until now, but having the class and the singleton instance live in the same namespace is really confusing to me.

Maybe:
```
paella.pluginRegistry.flexSkipPlugin = new paella.plugins.FlexSkipPlugin();
```
?