Add addEventListener() and removeEventListener() methods

[gruns]

Currently event handlers can only be setup at initialization time in the
constructor's options parameter.

Instead, iFrameResizer should implement the standard EventTarget interface

• https://developer.mozilla.org/en-US/docs/Web/API/EventTarget

with addEventListener(), removeEventListener(), and dispatchEvent() so 1)
iFrameResizer follows the standard JS events API and 2) event handling can
change dynamically.

[davidjbradshaw]

Do you want to make a PR?

[gruns]

Do you want to make a PR?

I'd love to, but I am starved for time right now. I won't be able to take a stab
at this for, at minimum, a few weeks.

To anyone with providential inclination: feel free to take a crack at this.

[davidjbradshaw]

I am on holiday for the next month, so no rush

[davidjbradshaw]

Roadmapped for v5.1

[bertho-zero]

That would be nice, we can't add onMessage after initialization at the moment