FR: Allow use of https://search.r-project.org refmans for autolinking

[TimTaylor]

Would you consider allowing users to specify the use of the help pages at https://search.r-project.org as opposed to https://rdrr.io/ (or perhaps any arbitrary alternative)?

E.g. it would be nice if we could make grep() documentation linked to

https://search.r-project.org/R/refmans/base/html/grep.html

instead of

https://rdrr.io/r/base/grep.html