When implementing `AssetLoader ` you need to specify which File extensions are supported by that loader.
Currently, Bevy always says it supports extensions that actually require activating a Feature beforehand.
This PR adds cf attributes, so Bevy only tries to load those Extensions whose Features were activated.
This prevents Bevy from Panicking and reports such a warning:
```
Jun 02 23:05:57.139 WARN bevy_asset::asset_server: no `AssetLoader` found for the following extension: ogg
```
This also fixes the Bug, that the `png Feature had to be activated even if you wanted to load a different image format.
Fixes#640
4fed2ee858