Add 'matrix' display for multi-sample VCFs

[cmdcolin]

This is an older effort and is still in a bit under construction but could be useful to push forward

example image

potential todos

• label rows
• sort rows
• group rows
• draw line from matrix position to genomic position, or just draw the lines at actual genomic position (these two use cases have different purposes)
• minor allele frequency filter to filter out low freq variants

functionality is similar to multivariantviewer (jb1 plugin) and igv

https://www.youtube.com/watch?v=ZKwm8dqIQpg

[cmdcolin]

Combined with some optimizations to vcf-js (GMOD/vcf-js#105) we can visualize large 500+kb regions of 1000 genomes dataset without too much trouble (example below takes ~10 seconds)