Provide packages to load before build reference

[hadley]

So that packages with dynamic documentation (e.g. dplyr) could automatically attached a couple of other packages (e.g. dbplyr and dtplyr).

[dmurdoch]

The rgl package doesn't need to load packages, but it would be nice if it could set some environment variables and/or options. So maybe this should be generalized to "run some code before building references"?

[hadley]

Maybe if pkgdown/pre-reference.R exists, it should be evaluated? Could add post-reference.R too, just in case you need to do any cleanup.