Added onload option

[Flushot]

Added an onload callback option that gets invoked after the script tag loads.

[vvo]

Hi! Thanks for contributing.

I'd rather add an option like this:

var analytics = require('analytics.js-loader')({
  onScript: function(scriptElement) {

  }
});

Then you could do anything on it like addEventListener etc..

Works?

[Flushot]

That works. I basically just needed an onload callback so that I could set a custom anonymous ID with window.analytics.user().anonymousId(foo) once their script loaded.