Added support for fading in and out.\nAdded support for loading and unloading multiple assets at once
Add 'vendor/howler.js/' from commit '84d6889233f959b85f7921f9af11a71950739eff'

git-subtree-dir: vendor/howler.js
git-subtree-mainline: 8871d7489f11b3919fb3ab20738e5509dc60b564
git-subtree-split: 84d6889233f959b85f7921f9af11a71950739eff