Add feature to show/hide bits of a guide based on preferences

Add support for embedded CSS classes that allow hiding segments of a guide based on build systems. Embed these selections in a cookie so that preferences are carried across various guides.
This block is duplicated in `reveal()`.  Maybe `reveal` could delegate to `hideBuildSteps` by calling `hideBuildSteps` before the `addClass` on line 85 (it seems safe to assume that `body.addClass` will not throw).

It'd also be cool to find way not to hardcode the `/guides/gs` path at this level.