Feature add custom emitter

[DanielMartinus]

With this pull request you're able to add your own custom emitter to Konfetti.

Custom Emitter

Extend from the following Emitter class to implement your custom emitter

/**
 * Created by dionsegijn on 9/03/17.
 *
 * An abstract class for creating a custom emitter
 * The only goal of the emitter is to tell when and how many particles to create
 */
abstract class Emitter {

    /**
     * Call this function to tell the RenderSystem to render a particle
     */
    var addConfettiFunc: (() -> Unit)? = null

    /**
     * This function is called on each update when the [RenderSystem] is active
     * Keep this function as light as possible otherwise you'll slow down the render system
     */
    abstract fun createConfetti(deltaTime: Float)

    /**
     * Tell the [RenderSystem] when the emitter is done creating particles
     * @return true if the renderSystem is not longer creating any particles
     *         false if the renderSystem is still busy
     */
    abstract fun isFinished(): Boolean
}

See BurstEmitter or StreamEmitter for examples

To start using your custom Emitter, call the following function in ParticleSystem and supply it with your custom emitter:

/**
 * Add your own custom Emitter. Create your own class and extend from [Emitter]
 * See [BurstEmitter] and [StreamEmitter] as example classes on how to create your own emitter
 * By calling this function the system wil start rendering the confetti according to your custom
 * implementation
 * @param [emitter] Custom implementation of the Emitter class
 */
fun emitter(emitter: Emitter) {
    startRenderSystem(emitter)
}

Calling your custom emitter will look something like this:

viewKonfetti.build()
    .addColors(Color.YELLOW, Color.GREEN, Color.MAGENTA)
    ... // other properties
    .emitter(CustomEmitter())