Support gradient shadow

[montanaflynn]

Replacing a solid background with background: linear-gradient(white, #cccccc);

[danielmahon]

@montanaflynn you can just replace the top and bottoms colors with whatever background you would like:

          <ScrollShadow
            shadowSize={8}
            topShadowColors={{
              inactive: 'white',
              active:
                'radial-gradient(at top, rgba(0, 0, 0, 0.2), transparent 70%)',
            }}
            bottomShadowColors={{
              inactive: 'white',
              active:
                'radial-gradient(at bottom, rgba(0, 0, 0, 0.2), transparent 70%)',
            }}
          >
            {content}
          </ScrollShadow>