Add output of discordant variants

I have added an optional --discordant option to write the discordant vcf records to a file.

Important

The default behaviour was to write the concordance stats stdout as json. This has been changed, and the concordance json file has to be explicitly specified with --stats.

Hi Redmar. Kun je de PR hier maken: https://github.com/lumc/vtools? Dan houden we die tool in het vrije software gebied.

Groet, Ruben

merged