Add ability to get R^2 coloring similar to locuszoom

[cmdcolin]

We have custom coloring in this plugin, but for a common visualization style implemented by locuszoom, the coloring is actually relative to clicking on the 'leading snp' using R^2 correlation with that feature

We could load PLINK R^2 output files, and allow this

[cmdcolin]

Note that early work in jbrowse allowed doing this using the ENSEMBL 1000 genoems ld api but its likely better to use some static file approach

[cmdcolin]

Example