New API for external plugins #1976

The current plugin API depends on AMD module system (via requirejs) and exposes internal modules. This blocks #1975 and causes potential breaking changes when any internal functions change.

We may introduce a new API that looks something like webpack one:

var respecConfig = {
   // ...
   plugins: [
     new MyPlugin()
   ]
};

@sidvishnoi @marcoscaceres Thoughts?