add option to skip building of vignettes about drat.builder

[richfitz]

I was thinking adding this as per-package as well, so one might write:

user/repo/subdir@ref --no-vignettes

or

user/repo/subdir@ref {"vignettes": false}

but your per-build option (021a4c6) is also a great idea.

[cboettig]

Where did you get to on this? Does the above notation work? I see https://github.com/richfitz/drat.builder/blob/master/R/drat_builder.R#L468 but it's not obvious to me how to configure that.

For my use case I imagine toggling it globally (e.g. I generate ropensci.txt list programmatically from the ropkgs API, because I wouldn't necessarily know when a new package is added to ropensci github or whether or not the vignette shouldn't be build)

[cboettig]

@richfitz hmm, this doesn't seem to be working yet. It looks like travis is deceiving you -- it says build passed but looking on the logs it isn't working there either, unless I'm missing something.

[richfitz]

I think that this is working (checked in tests) - can you check?