Pausing between keypress input

[zcorpan]

As part of #26 we discussed supporting pausing, which WebDriver also supports.

WebDriver handles pauses as part of the Actions API, which also provides low-level control of keyboard, pointer and wheel input sources. (The waiting happens in https://w3c.github.io/webdriver/#ref-for-dfn-dispatch-actions-inner-1 )

I figure the message structure could be something like this, for only sending a "pause" message with 200ms duration, which could be extended to support other input sources in the future:

{
  "method": "actions.perform",
  "params": {
    "actions": [{
      "type": "none", // input source
      "items": [{
        "type": "pause", // subtype
        "duration": 200
      }]
    }]
  }
}