Feature: Variant annotations


Hi everyone,
  This is a pull request for adding an Avro IDL file for variant annotations (annotations.avdl). 
  Annotations include records for variant effect, impact, coding and non-coding information, 
clinical relevance, etc.
  It is heavily based on VEP (ENSEMBL's Variant Effect Predictor) and SnpEff (an annotations 
tool I wrote).

Yeah, it'll be a comment. This would most closely match the [GASearchReadsResponse.alignments field](https://github.com/ga4gh/schemas/blob/master/src/main/resources/avro/readmethods.avdl#L61)