Heatmap log scale

**Is your feature request related to a problem? Please describe.**
I have a heatmap with a huge range of values, and I'd like to see the colormap in the log scale.

**Describe the solution you'd like**
Add an option `colormap_scale='log'` to heatmap opts.

**Describe alternatives you've considered**
Maybe it can be already done with `opts.layoutopts` but I don't know how.
