add support "percentage" value for "column-gap"

[horans]

when using a percentage value for column-gap, i received the known-properties warning.

dl { column-count: 2; column-gap: 18%; } /* csslint allow: known-properties */

references: https://drafts.csswg.org/css-multicol-1/#cg https://developer.mozilla.org/en-US/docs/Web/CSS/column-gap