ENH: Simple reversion coloring to allow inspection of interesting/problematic parts of tree

[corneliusroemer]

Context
It would be great to have a simple reversion coloring that counts number of reversions from the root till the node.

This should be easy to script and add, would help a lot with a lot of the work I do on Nextclade trees but should also be useful in general, helping identify problematic parts of trees (e.g. tree builder error), recombinants, quality issues etc.

This script may well end up so useful that I could envisage adding it to augur!