Allow custom keydown handler to be attached

[Tyriar]

Fixes #118

---

Example usage:

// Don't handle any events
this.xterm.attachCustomKeydownHandler(function (ev) {
  return false;
});

// Handle everything except ctrl+m
this.xterm.attachCustomKeydownHandler(function (ev) {
  return !(ev.ctrlKey && ev.keyCode === 77);
});

[parisk]

Could you please change !this.customKeydownHandler(ev) with (this.customKeydownHandler(ev) === false) in order to require users to explicitly return false to stop further handling of keys?

[parisk]

@Tyriar could you rebase with master please? I think we can move on with merging this afterwards.

[Tyriar]

Looking into test failure