Explanation of the mixin:
The mixin basically generates a gradient container
and two triangles that are used to block out the shape.
@include triangle($half-width, $height, $color-a, $color-b, $direction)
$half-width:
half of the total width
$height:
height of the triangle
$color-a/b:
colors used in gradient
$direction:
direction of trangle: up, down, left, right