Features
---------
* Added an option (--json-keys) to specify select keys that should be included in the JSON output. Useful if you only want a subset of the default JSON output file. Argument should be a comma-separated list of key names (v_gene,d_gene,j_gene,cdr3_aa)
Bugfixes
---------
* Fixed a bug when attempting to use a custom germline database that was named something other than one of the built-in databases