Add gene coverage to MultiQC report

[Redmar-van-den-Berg]

It would be nice to get an overview of the (exon) coverage of the target genes directly in the MultiQC report, rather than relying on the current (10x, 20x etc) coverage table that is part of the pdf report.

This can be achieved by combining the MultiQC additional data and custom content features to directly inject the coverage information into the report.

Example JSON and report file:
example_gene_coverage.zip