Instant search suggestions

[gaborluk]

This changeset implements instant search suggestions per #32.

To receive suggestions, the user has to explicitly type the extension's keyword first (currently d), and hit space. If the d keyword is already defined in the browser as another search provider's keyword, the omnibox will not enter "extension mode". The implementation emulates Chrome's inverse suggestion highlighting style (the suggested part is bold), and handles !bangs.

It would be desirable for the omnibox to display instant suggestions as soon as the user types something into the naked omnibox, but currently this is not doable. While the manifest.json documentation specifies an optional instant_url key for the search_provider, it is not documented how this should work or what format Chrome expects the suggestions to arrive in, and is probably not even implemented, much like the related search_url_post_params.

[jdorweiler]

@gaborluk Thanks for the PR! This looks really interesting. We're thinking of making some changes to our browser extensions so we will need to review this and see if it will fit with our plans. I'll leave this open for now and get back to you when we figure that out.

[gaborluk]

@jdorweiler, thanks for the heads-up. As for the changes, if you need a helping hand, let me know.