Add ability to use CSI indexes in @jbrowse/img

[cmdcolin]

Let's you specify a command like this

--bam myfile.bam index:myfile.bam.csi --gffgz myfile.gff.gz index:myfile.gff.gz.csi --bedgz myfile.bed.gz index:myfile.bed.gz.csi

it could potentially auto-detect whether the csi index exists, but this might be best done in the adapter itself

[cmdcolin]

this also removes the "MUI emotion cache" from @jbrowse/img. there was some vague comment that it helped with lgv headers but didn't appear truly needed.