set default effect properties in HowlerGlobal.prototype...

... instead of overriding HowlerGlobal.prototype.init()

The HowlerGlobal object (window.Howler) is instantiated here:
https://github.com/goldfire/howler.js/blob/2.0/src/howler.core.js#L232

Before HowlerGlobal.prototype.init is overrided here:
https://github.com/goldfire/howler.js/blob/2.0/src/plugins/howler.effects.js#L25

Effect properties were not set for the Howler global object (ie: window.Howler)