enhancement: the rias plugin doesn't support {height}

[LRancez]

Hi there,
I really like lazysizes. Great work, thanks for sharing!
But the rias plugin don't currently work with the {height} replacement.
Some automated resize systems require the height. Besides, it will be very useful for applying automated cropping. I needed this {height} to make cool crops with thumbor, for example.
So I'd made a PR with the needed changes: #213

So far is working good, but don't hesitate in let me know if you have any improvements to that code.
Best,

Lucas

[LRancez]

Done: #213