Hi bred!

I have a suggestion for your already amazing vcfanno.

When annotating AF from an external file (gnomAD for example) it would be more convenient to have the annotation field set to zero for variants not found in gnomAD VCF.
In this way, one can simply filter for rare variant doing gnomAD_AF < 0.01 for example

In general, it would be great to have the possibility to specify in the toml configuration file how to annotate variants when they are not found in the annotation source, so one can decide to leave them un-annotated (as it is right now) or adding a value (like zero for AF annotation or some custom value for other kinds of annotations).