add custom events listeners

[jonathanargentiero]

run functions registered under custom names which doesn't match with begin or end.

example:

Events.scrollEvent.register('fn1', this.functionOne.bind(this));
Events.scrollEvent.register('fn2', this.functionTwo.bind(this));

this change address to the possibility to handle different instance of react-scroll in the same view or having multiple listeners which can be unsubscribed at different moments.

[webdevtrev]

Hello @jonathanargentiero how do you assign these custom events to begin or end? or is that possible? I'm struggling to know when these will trigger.