support reversed color schemes

It would be awesome if color schemes could be inverted, e.g.,

from the red sea example. In the picture, I am using redblue color scheme, but due to ordering, colder temperatures are red and warmer temperatures are blue, which is a little counter-intuitive and it would theoretically be nice to display the legend with the opposite color ordering. There is no bluered color scheme in vega-lite, so in order to get the color scheme inverted, you have to invert the upper and lower bound of the domain, something like adding domain=[1, 0] to the scale call:

https://github.com/biocore/qurro/blob/a37037bdf83770349e1e05f363692c8c202db173/qurro/generate.py#L289

I think user-side, this would look something like having a checkbox titled Invert quantitative color scheme, that you can use to apply the domain change.

This issue https://github.com/altair-viz/altair/issues/1068 may be informative