call onLoad given as Props

[yukukotani]

Currently onLoad is overridden by handleLoad so does not works.

[ryanseddon]

We might need to rejig this once #207 merges as the logic moves away from onLoad to utilising DOMContentLoaded instead

[yukukotani]

@ryanseddon oh I didn't notice that.

Or maybe we don't need this. We manually subscribe load event by addEventListener so it seems there is no reason to give onLoad props of iframe. Can we just remove onLoad props at line 139?

[yukukotani]

@ryanseddon
Sorry please forget the comment above, i misunderstood the logic.

I've added the commit 85b924e so that this PR won't conflict with #207.