Respect the html parameter in trackDb file

Created by: brucebolt

I have noticed the UCSC TrackHub specification has a html parameter for the description file URL (see https://genome.ucsc.edu/goldenPath/help/trackDb/trackDbHub.html#html), which appears not to be used by Ensembl. Instead, Ensembl generates its own description URL based on the track name.

This modification looks for a html parameter before using Ensembl’s default assumed URL. It also deals with inheritance of the parent's description if a parent value is specified.