Added a mixin for breakpoints.
Added a mixin for breakpoints.

hellfish2/davidensinger.github.io,davidensinger/davidensinger.github.io,S1SYPHOS/davidensinger.github.io,S1SYPHOS/davidensinger.github.io,hellfish2/davidensinger.github.io,S1SYPHOS/davidensinger.github.io,davidensinger/davidensinger.github.io,davidensinger/davidensinger.github.io