Accept `allowedModifiers` array on register

[patrickberkeley]

Stemming from #9.

The following example would create shortcuts for control+a, alt+a, shift+a:

keyManager.register({
  executionKey: 'A',
  allowedModifiers: ['shift', 'alt', 'control'],
  callback: () => {
    //
  },
  type: 'keydown',
});