Add support for synthetic events (CustomEvent)

[meteficha]

Synthetic events use CustomEvent(), which contains a customizable detail attribute. This PR allows that attribute to be passed to the Haskell side, allowing one to, say, use:

on (domEvent "myEvent") elm $ ...

on the Haskell side, and:

var ev = new CustomEvent("myEvent", { detail: ... });
elm.dispatchEvent(ev);

on the JavaScript side.

Cf. https://developer.mozilla.org/en-US/docs/Web/API/CustomEvent/CustomEvent

[ndmitchell]

This seems very useful, anything blocking its review?

[HeinrichApfelmus]

This seems very useful, anything blocking its review?

A lack of leisure time? 😄🖇

I wasn't quite sure how this affects marshalling event data. Apparently, it doesn't, event data is still JSON.

Merged, thanks!

[meteficha]

Wicked, thank you!