pass pkgdown.internet option in build_site_external

[cderv]

This fixes #861 by passing the option pkgdown.internet that could be set by the user to build_site_external call to callr::r(). It completes #774.

This now works:

options(pkgdown.internet = FALSE)
pkgdown::build_site()

I did not look yet if I should add a test for build_site_external.

[jayhesselberth]

Looks good to me. Thanks!

[cderv]

About the error in Travis, it does not seem related to this PR.

About NEWS.md, I did not added a bullet as there is already one about the option pkgdown.internet. Should we update NEWS with the PR number only ?

Users with limited internet connectivity can explicitly disable pkgdown CRAN checks by setting options(pkgdown.internet = FALSE) prior to running build_site() (#774, #877)

About test, I looked into it but I did not found where to test and how because it is really specific. If you want some test, please advice.

Finally, I wanted to add further documentation about internet connectivity requirement to explain that if the package for which pkgdown is used requires internet connection for example or vignette, this has nothing to do with pkdown.internet option and it won't works offline.
I did not find where to put that. There is an internet section in build_home but not build_site.
Would you like me to add this ? In a new section in build_home or addition to the existing one ? In the documentation of build_reference and build_articles ?

Thanks.

[hadley]

Yes, that's the right strategy for the news bullet.

For the docs, maybe move the existing section from build_home() to build_site() and expand? That might be better in a separate PR.

Thanks!