Add rseek search engine

[rossellhayes]

Adds rseek as a search engine and adds associated function search_rseek().

[coatless]

@rossellhayes thanks! By chance do you know what websites are scanned by the custom Google search rseek provides? I'd like to include that in the documentation entry.

[rossellhayes]

@coatless That's a great question. Unfortunately, I haven't been able to find an about page with that kind of info. What I can say is that it's using a custom Google search and that it returns results from a pretty wide array of sites, so it either maintains a really expansive list of R related sites or it's appending some targeted keywords. I can reach out to the maintainer and see if I can get some more info though!

[coatless]

@rossellhayes perfect, thanks!

From a quick glance, I saw a short list on the old Revolutions blog but that wasn't exhaustive.