allow organization of search engines on index.html by topic (specified in document_sets.json)

[jeremybmerrill]

Right now, search engines can be organized only by sort order. ("1" goes to the top, "2" goes underneath, sorting of search engines with the same sort order is not guaranteed.)

It would be nice to create the (optional, potentially) functionality for categorizing each search engine by a category, e.g. "archives" or "politics" or "international" or "wikileaks". On index.html, teh search engines would be grouped by the category and displayed together.

You could probably add a "table of contents" at the top of index.html.

[jeremybmerrill]

closin' tickets or ticketing closes amirite? https://github.com/newsdev/stevedore/commit/057e043e1b953b022250069d3cef5d014071f03a