Add a stretch=false option to prevent image from being stretched fullscreen

[augnustin]

Great plugin! Thanks

If the image is smaller than the screen, it still gets stretched up to the screen size, hence appears pixelized.

I removed this by having:

<expandable-image class="no-stretch" :src="img"></expandable-image>

<style>
  body > .expandable-image.expanded.no-stretch > img {
    width: auto;
    height: auto;
  }
</style>

This should definitely be an attribute of the component, with stretch="true" being the default behavior for backward compatibility (though I doubt this is actually the most useful case).