Add support for linear gradients to be used as colors for slantRectangles

use the following syntax: linear-gradient(to [direction], #color stop%, rgb(r,g,b) stop%)

For the most part, this should follow the CSS3 spec; note the following: first and last stop% can be ommitted. [to direction] can be ommitted (defaults to 'to bottom'). Color will accept HTML colors, Imagick colors, hex colors, and rgb() colors. Stop distances must be in %s (would pixels be useful??).