Recent and upcoming features for Handbook, design standards

With all the features going in to support the 18F/handbook design, I thought I'd enumerate them here:

[X] #43: Breadcrumbs implementation (repeated in #44 thanks to a botched force-push on my part). Already submitted, but I've not yet added an HTML fragment to the guides_style_18f templates.
[x] #45: Flat namespacing, whereby the permalinks corresponding to the directory structure of the _pages collection are flattened such that every page is a direct child of {{ site.baseurl }}, e.g. /about-us/offices/new-york-city/ becomes /new-york-city/.
[x] #46: Automatically generated nodes in the navigation: list in _config.yml for directories, e.g. for /about-us/ and /about-us/offices/, or for /offices/ in the presence of flat namespacing (#45).
[x] #47 Generated pages from navigation: nodes marked with generated: true. The default is to generate a page with a meta redirect to an ID on the home page, e.g. /offices/ will redirect to /#offices.
[x] 18F/jekyll_pages_api_search#15: Removes dependency on Angular, and generates a search results page rather than a dynamic drop-down.
[x] #48: Incorporates the revamped search page implementation from 18F/jekyll_pages_api_search#15.
[x] 18F/handbook#566: Integrates support from #43 (and #44), #45, #46, and #47 into the Handbook.
[x] 18F/handbook#567: Depends on 18F/handbook#566 and integrates support from #48 into the Handbook.

The screenshots below show the changes in action in both 18F/guides-template and 18F/handbook.

cc: @andrewmaier @mtorres253 @ccostino @ertzeid @catherinedevlin @nicoleslaw

handbook-demo

guides-search-demo

18F / guides-style