Fix BED track for UCSC Genome Browser

Previously, the 'color' field in the BED track we provide for the Genome
Browser was set to the empty string, but Gerard informed us that this is not
(always) handled correctly:

   Expecting number field 5 line 2 of https://mutalyzer.nl/bed?..., got +

We now explicitely set the 'color' field to '0'.


git-svn-id: https://humgenprojects.lumc.nl/svn/mutalyzer/trunk@439 eb6bd6ab-9ccd-42b9-aceb-e2899b4a52f1