Add typeahead algorithm for <select>

[dandclark]

Add typeahead behavior to the <select> anatomy.
This behavior allows a user to choose items in the listbox by typing characters that match the first letters of a given <option>. After a short timeout, the search "buffer" is cleared, and the user can begin a new search.

This is distinct from the "filtering" approach associated with comboboxes that have a textbox and listbox, which we haven't yet included in the anatomy. See discussion in #104 and #145.

Closes #145.

[dandclark]

For the review, I'm interested in feedback not just on the algorithm but also on how I've presented it.
This is more complex than other behaviors described in the anatomy so far, so I didn't have a precedent
to work with. I ended up writing it in a similar style to parts of the HTML spec. Additionally I used JS
functions to describe the array manipulation in the typeahead steps. I could alternatively do that part
in prose, or define slice/concat as utility functions in OpenUI; not sure what is clearer.

[gregwhitworth]

I'm saying request changes since I left a bunch of comments/questions that I'd like to discuss prior to completion

[dandclark]

(Force-pushed to resolve merge conflicts).