`...` missing from ggplot function calls

[markjrieke]

Howdy! The package documentation notes that the arg ... can be used to pass arguments to underlying ggplot2 functions (scale_*_gradientn() and discrete_scale()), but looking at the function internals, this is actually missing (found this out when trying to get the limits arg for scale_color_gradientn() to work). It should be a pretty easy add --- let me know if you'd like me to open a PR