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:empty {
  background-image: url(atom://assets/my-background.png);
  background-repeat: no-repeat;
  background-position: center;
  background-size: 90% 90%;
}
```

And have it be portable across machines and easily stored in a dotfiles repo.


Fixes #4010