Add remote BED, VCF, Bigwig file support

[saliksyed]

• [x] Look in to js-seq-tools for reading the BED, 23andMe, VCF, BAM data clientside: https://github.com/jason-weirather/js-seq-tools
• [ ] Implement a file data resource class, which given either a remote or local file + format, will return the appropriate data
• [ ] for 23andMe tracks, we may need some type of logic to convert to variants (it's a list of rs numbers and allele values for those rs numbers, perhaps we even want to create a subclass of VariantTracks called AlleleTracks that show the actual alleles present?): https://samtools.github.io/bcftools/howtos/convert.html
• [ ] Add either a variant (VCF, 23andMe) or interval (BED) track that utilizes remote data.
• [ ] Extend the interval track so that clicking works (you should display the BED metadata when the interval is clicked)
• [ ] hook up the UX in the add track UX so users can paste in their own remote URLs or browse from their hard disk

[saliksyed]

@haxiomic : threw some initial tasks together, let me know if you have comments/feedback, and feel free to edit directly.

[saliksyed]

NOTE: These will be subsumed by new tasks within the genome browser modularization task