SeleniumAppQuery doesn't implement IAppQuery.Index()

[davidjohnoliver]

I'm submitting a...

• Feature request

Minimal reproduction of the problem with instructions

		private void ClickCheckBoxAt(int i)
		{
			_app.Marked("CheckBox").AtIndex(i).Tap();
		}

Current behavior

Throws NotImplementedException

Expected behavior

Taps i'th checkbox

Environment

Package Version(s): 

Visual Studio
- [ ] 2017 (version: )
- [ ] 2017 Preview (version: )
- [ ] for Mac (version: )