Add responsive borders

Created by: jnsdrtlf

Changed syntax for the border utilities to add or remove borders depending on the screen size. Using the notation from the spacing utilities like margin or padding, you can now provide a breakpoint to the additive and substractive border classes. This might be useful for dividers that should only be visible on large screens.

Examples:

• border-left applies to all screen sizes
• border-md-left for a border at the medium breakpoint and above
• border-sm-0 to remove a border at the breakpoints small and above