Control available scene modes in SceneModePicker

The [BaseLayerPicker](https://cesiumjs.org/Cesium/Build/Documentation/BaseLayerPicker.html?classFilter=BaseLay) allows you to pass an array of which imagery layers should be available. It would be nice if the [SceneModePicker](https://cesiumjs.org/Cesium/Build/Documentation/SceneModePicker.html) had a similar option.

This was [requested on the forum](https://groups.google.com/d/msg/cesium-dev/2eUX-TdtzHM/82m3_m1LAwAJ). Currently it's hardcoded to show all 3. I think it should be easy to add an option to control this. I think you'd need to edit https://github.com/AnalyticalGraphicsInc/cesium/blob/master/Source/Widgets/SceneModePicker/SceneModePicker.js and https://github.com/AnalyticalGraphicsInc/cesium/blob/master/Source/Widgets/SceneModePicker/SceneModePickerViewModel.js.

Start by reading Source/Widgets/SceneModePicker/SceneModePicker.js and Source/Widgets/SceneModePicker/SceneModePickerViewModel.js, where the three scene modes are currently exposed. Trace how BaseLayerPicker accepts its available options and determine how a similar option should control SceneModePicker. Done means callers can restrict the available scene modes instead of always seeing all three.

Written by the indexing model from the issue text.