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.
I think this conditional dispatch could be simplified by keeping a hash of the supported names, and simply checking if the hash contains the `preference` as a key.  Here's an example (suggested by @unscriptable via hipchat):

```js
if (preference in buildOpts) {
    reveal(preference);
} else {
    hideBuildStep();
}
```

where `buildOpts` is the hash of supported types.