Prevent default event action

[fxha]

Add support to prevent the default action when the callback returns true. E.g:

electronLocalshortcut.register(win, accelerator, () => {
  // ...
  return true // prevent default action
})

---

fixes #69

[ibc]

Never merged, right?

[johnnybenson]

Curious why not pass e to the callback and let the downstream code preventDefault? That feels more "normal" like working with typical event handler callbacks.

[fxha]

I created this from our use case and we always return true to prevent Electron from handling events (c&p from our fork). I also prefer the cleaner control flow and don't think it's necessary to pass the whole event, but on the other side, others may require the event.