Extend shadows to accept spread

[angelikatyborska]

The current shadow generation tool accepts only offset, blur, and color: https://github.com/bitcrowd/bitstyles/blob/724581e96368156327cc5680f7243e4fe3f43da8/scss/bitstyles/tools/_shadow.scss#L72-L76

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?