added event emitter module

[sc0ttj]

• new feature: added an event emitter
  • if emitter is installed, component "actions" will emit an event
  • other components can listen to it with myComponent.on('actionName', (props) => { ... }
  • props will contain the latest state of the component that emitted the event
• added src/emitter.js, implemented as an optional, extra module
• updated build process to also build dist/emitter.min.js
• added examples and updated README
  • added examples/usage-emitter.js