Load atom://assets/ urls from ~/.atom/assets

Maps URLs like `atom://assets/my-background.png` to `~/.atom/assets/my-background.png` so you can put something like this in your `~/.atom/styles.less`:

```css
.item-views {
  background: url(atom://assets/my-background.png) no-repeat center center;
  background-size: contain;
}
```

And have it be portable across machines and easily stored in a dotfiles repo.


Fixes #4010