* added `output=vcf` option for variant export & made it default for the
`phenopackets` entity
- VCF export is basic & hasn't been tested for round trip compatibility
* added filter exclusion flag:
- for POST a Boolean `"excluded": true`
- for GET prefixing a term by an exclamation mark (e.g. `!PATO:0020002`)
- this is a **BeaconPlus** feature - see [issue 63](https://github.com/ga4gh-beacon/beacon-v2/issues/63) in `beacon-v2`