Add output column with SNV type

[caspargross]

It would be helpful to identify the type of SNV found.
In cfDNA we have three types: Monitored (exptected SNV target), ID (Fingerprint SNP, KASP) and Background (in target region but not at expected position).

The easiest way would be to add an optional input .bed file with SNV positions and an additional column that specifies the type (0 = monitored, 1 = ID)