Per-step wait support

[heydenberk]

While testing a rich web app with lots of DOM updates that don't trigger new page loads, I find myself doing a lot of this in my "steps":

{
  "type": "waitForElementPresent",
  "locator": {
    "type": "id",
    "value": "someId"
  }
},
{
  "type": "clickElement",
  "locator": {
    "type": "id",
    "value": "someId"
  }
},
{
  "type": "waitForElementPresent",
  "locator": {
    "type": "id",
    "value": "anotherId"
  }
},
{
  "type": "clickElement",
  "locator": {
    "type": "id",
    "value": "anotherId"
  }
}

I'd like to be able to specify a "globalWait" on the top level of the JSON, or a "wait" parameter per step. On my fork, I hacked together a version of the interperter that works this way, but I'm not comfortable issuing a PR unless there is some interest in implementing this feature. I'm not sure how it would interoperate with existing "waitFor" step types

Thanks and let me know what you think!

[Zarkonnen]

You shouldn't have to do the waits before the click! Webdriver, which se-interpreter is built on, uses implicit waits, so the playback should wait for the appearance for the element anyway. Will double-check this works in the generic case.