Respect user-supplied color range given zero-variance data

[mdoucleff]

Thank you for publishing the reactablefmtr package. It has helped me achieve a professional look to my app. I've encountered two challenges with the fixed color ranges and would like to share a fix.

First, the zero-variance checks cause user-supplied color range values (min_value & max_values) to be ignored. Note that zero-variance will always occur for single-row tables. This changes applies the zero-variance test after selecting the color range end points instead of before. The variance test itself is also reduced down to a simpler range calculation.

Second, in some fixed color range applications (e.g. red for negative, green for positive) the potential data range is unbounded, which limits the usefulness of hard "min_value must be less than the minimum value observed in the data" stop's. So this change removes the stops and instead clamps the post-normalized value to [0,1]. That way data values outside [min_value, max_value] are displayed using the most-extreme color value.

A different approach (not included in this change) to scale-and-clamping would be to map the potential (-Inf,Inf) data range to the (0,1) color range via some sigmoid function. That way there is always a more-extreme color for a more-extreme data value in a continuous way (although becoming infintesimally similar). Since there are endless such mapping functions, consider accepting an arbitrary mapping function as input, with scale-and-clamp and sigmoid function-factories provided by the package. Just an idea...

Fixes kcuilla/reactfmtr#43