More flexible configuration of vignette building

[gaborcsardi]

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For package trees. It should be possible to supply a vector of package names, to build vignettes for these packages only. Or negated packages names, e.g. "!pkgx", and then build vignettes for all packages except for the negated ones.

[gaborcsardi]

This does not seem very important currently, but eventually we can have a ?novignettes and a ?vignettes parameter, these are pretty simple to implement.