feat: added new option --gradient and --gradient-angle

[mazznoer]

Added support for creating color gradient as a background. It uses CSS gradient format.

Examples

Default gradient angle is 45 (top left to bottom right)

silicon --gradient 'dodgerblue, darkslateblue, crimson'

Set gradient angle to 0 (left to right)

silicon --gradient 'dodgerblue, darkslateblue, crimson' --gradient-angle 0

Using another color formats.

silicon --gradient 'seagreen, #b4dA55, rgb(0,125,200)'

silicon --gradient 'lightskyblue, darkslateblue'

Complex gradient.

silicon --gradient 'tomato, gold 70%, hotpink 70%, purple 85%, red 85%, khaki'

With shadow