Added support for overlay sprite arrangement

When using overlay arrangement, it's possible to use the sprites without a `viewBox` argument, since all sprites will be at coordinates `0 0`. The size can then be defined via CSS alone.

    <style>
        .icon { height: 32px; width: 32px; }
    </style>
    …
    <svg class="icon">
        <use xlink:href="path/to/icons.svg#my-id"></use>
    </svg>