[Rivet] mkhtml config options

[David Grellscheid]

Hi Frank,

> does the second option described here:
> https://rivet.hepforge.org/make-plots.html#_configuration_files
> not work anymore?

Ah, thanks! That's what I was remembering. But it only works in 
make-plots, not in rivet-mkhtml. We'll need to fix that.

See you,

   David