Extend shadows to accept spread

[angelikatyborska]

The current shadow generation tool accepts only offset, blur, and color:

bitstyles/scss/bitstyles/tools/_shadow.scss

Lines 72 to 76 in 724581e

	$layer: (
	'color': $color,
	'offset': $layer-offset,
	'blur-radius': $layer-blur-radius,
	);

The CSS property box-shadow can also have a spread value. The shadow generation tool is not able to handle that currently.

Please note that the second usage of shadow generation is the drop-shadow filter which does not support a spread value. How to handle that?