Allow Skybox to take a Color (#57)

This PR Adds the ability to set the color of the skybox using the URL
parameters.

- Now you can pass a color's hexadecimal number (without the #) in the
  skybox parameter, and that will set the skybox to that color.

studio/src/lib.rs

@@ -136,6 +136,8 @@ pub enum InitAsset {
     /// Initialize the application by retargeting a Mixamo animation onto the
     /// bundled `Mannequin.gltf` model.
     MixamoAnim(String),
+    /// Initialize the application by loading an arbitary model given an identifier
+    MediaAsset(String),
 }
 
 pub fn start(init: InitParams, mode: StudioMode, window_config: Option<Window>) -> App {
@@ -231,5 +233,6 @@ fn init_from_asset(app: &mut App, asset: InitAsset, skybox_id: String) {
                 ew.send(ImportSceneEvent(scene.clone()));
             });
         }
+        InitAsset::MediaAsset(_) => todo!(),
     }
 }