devtools should include pkgs in VignetteBuilder as dependencies to download

[kevinushey]

For example, currently installing Rcpp from GitHub fails if highlight is not available:

install_github("Rcpp", "RcppCore")

gives me

* creating vignettes ... ERROR
Error in loadVignetteBuilder(vigns$pkgdir) : 
  vignette builder 'highlight' not found

Of course, the error is easy enough for the user to fix, but it would be nice if devtools could automatically grab the package from CRAN.

[hadley]

Should these packages be installed if deps = NA, or just if deps = T?

[hadley]

Here's one interpretation: deps = NA means (approximately) get run-time dependencies, and deps = T means get build-time dependencies. So it should be added only when deps = TRUE, but install_github() should probably use deps = T since you have to build the package.

[wch]

I think the TRUE/FALSE/NA values aren't very intuitive. An alternative is to change dependencies to a string argument, and you could deprecate the TRUE/FALSE values.

[hadley]

They're not intuitive, but they're standard for install.packages