Declare deepvariant optional html output

[peterpru]

When using this module with commands related to the deepvariant postprocessing, it will output an html file, however, the html file cannot be picked up by nextflow as it is not declared in the module, this PR fixes that.

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Closes #XXX

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[ramprasadn]

Nice! I think it be would good to update at least one of the tests to generate a html report.