adding calculation for carousel width in breakpoints

The breakpoints in slick are better suited for Walmart.com carousels if
we let them adjust based on carousel width in addition to window width.
adding responsive target settings

with default being window, and options slider and min as well.
adding slider and window width vars
Merge pull request #766 from xitongzou/add-carousel-width-calc

adding calculation for carousel width in breakpoints