add showAllBases documentation to the alignment track configuration documentation

[VidJa]

One of my co-workers asked to switch on "Show all bases" by default for an alignment track.

I found it worked to add showAllBases: true, to the config for browser.loadTrack(config) after digging into the source code(js/bam/alignmentTrack.js)

The documentation has no reference for this alignment track config option.
https://igv.org/doc/igvjs/#tracks/Alignment-Track/#configuration-options