Gradient mixin to allow nth number of colors and stops

Created a mixin that would allows nth number of colors and stops to be be used. Enables other developers to not have to use only 2 or 3 colors, instead they can use as many colors as desired.

Added the following mixin's:
- vertical-nth-colors
- horiztonal-nth-colors

Sample Usage:
```
#gradient > .horizontal-nth-colors(#ffffff #fffffa #ff0011 #000000 #affd29, 0 50 75 80 100);
#gradient > .vertical-nth-colors(#1E5799 #1A4884 #2672BF #7db9e8, 0 30 60 100);
```

It maintains backwards compatibility by having the current horizontal, horiztonal-three-colors, vertical, and vertical-three-colors use the same iterator pattern