Add nested kbd element styles for actual keys or inputs

> When the _kbd_ element is nested inside another _kbd_ element, it represents an actual key or other single unit of input as appropriate for the input mechanism.

http://www.whatwg.org/specs/web-apps/current-work/multipage/text-level-semantics.html#the-kbd-element

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This will maintain visual styling when using the semantic markup for individual hotkeys or other input controls (i.e. nested `kbd` elements).