Fix `RON` formatting for Amethyst

[conectado]

When trying to load an sprite sheet generated by the CLI using:

    let sheet_handle = {
        let loader = world.read_resource::<Loader>();
        let sheet_storage = world.read_resource::<AssetStorage<SpriteSheet>>();
        loader.load(
            "sprites/sprite_sheet.ron",
            SpriteSheetFormat(texture_handle),
            (),
            &sheet_storage,
        )
    };

I was getting and error due to the formatting of sprites/sprite_sheet.ron

[ERROR][amethyst_assets::progress] Error loading handle 6, Mesh, with name sprites/sprite_sheet..png: Asset was loaded but no handle to it was saved.
caused by: Asset was loaded but no handle to it was saved.
Error { inner: Inner { source: None, backtrace: None, error: UnusedHandle } }
[ERROR][amethyst_assets::progress] Note: to handle the error, use a `Progress` other than `()`
[ERROR][amethyst_assets::storage] "renderer::SpriteSheet": Asset "sprites/sprite_sheet.ron" (handle id: Handle { id: 0 }) could not be loaded: Failed to load asset with name "sprites/sprite_sheet.ron"
[ERROR][amethyst_assets::progress] Error loading handle 0, renderer::SpriteSheet, with name sprites/peakbeak.ron: Failed to load asset with name "sprites/sprite_sheet.ron"
caused by: Failed to load asset with name "sprites/sprite_sheet.ron"
Error { inner: Inner { source: Some(Error { inner: Inner { source: Some(Error { inner: Inner { source: None, backtrace: None, error: LoadSpritesheetError(Parser(ExpectedIdentifier, Position { col: 1, line: 1 })) } }), backtrace: None, error: Format("SPRITE_SHEET") } }), backtrace: None, error: Asset("sprites/peakbeak.ron") } }
caused by: Format "SPRITE_SHEET" could not load asset
Error { inner: Inner { source: Some(Error { inner: Inner { source: None, backtrace: None, error: LoadSpritesheetError(Parser(ExpectedIdentifier, Position { col: 1, line: 1 })) } }), backtrace: None, error: Format("SPRITE_SHEET") } }
caused by: Failed to parse SpriteSheet
Error { inner: Inner { source: None, backtrace: None, error: LoadSpritesheetError(Parser(ExpectedIdentifier, Position { col: 1, line: 1 })) } }
[ERROR][amethyst_assets::progress] Note: to handle the error, use a `Progress` other than `()`

When comparing the ron generated by the cli with the ron provided by Amethyst's examples I found out that the Sprite struct was wrapped with a List enum. So by doing the same with the output of AmethystFormat I managed to fix this problem.

Note: This PR includes a fix of #32 which I realize is fixed in #33 but was never merged and is currently outdated. If needed I can make a branch including only 436bec5 .