Add more responsive border utilities

[shawnbot]

This adds the following responsive border utilities:

• .border-{breakpoint}-0 removes borders on all sides responsively, a la .border-0.
• .border-{breakpoint} adds borders on all sides responsively, a la .border. I had this as .border-{breakpoint}-1, but removed the -1 suffix at @simurai's recommendation.
• For consistency, I've also added .border-x for horizontal borders only.
• The docs explain how all of the permutations work.

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Closes #756

[simurai]

Made a suggestion to change the example. And not sure about the -1. Otherwise 👍 .

[simurai]

This looks good to 🚢 👍 .