add `thinColor` style for bigBed tracks

[benwbooth]

When I load bigBed tracks into JBrowse, I noticed that the utrColor style setting is not used. Instead, the entire feature is the same color, unlike the ProcessedTranscript glyph. This is because bigBed uses the UCSC/Bed glyph instead, which does not use this feature. It would be nice to have a utrColor or thinColor style attribute to color the UTRs in bigBed tracks. Thanks!

[cmdcolin]

Maybe this is a different issue but I was kind of in the preference of transforming the feature into proper subfeatures in the store class rather than doing view side transformation, reason being is that the data model of the subfeatures like exon and CDS seems better and then we automatically get behavior like utrColor

[benwbooth]

It would also be nice if there was an option to use the ProcessedTranscript glyph with bigBed tracks instead of the UCSC/Bed glyphs. Thanks!