release candidate

• spvcf encode: perform QC squeezing by default (opt-out)
• rewrite header e.g. ##fileformat=VCF4.2 to ##fileformat=spVCFvx.y.z-gabcdef;VCF4.2 and back
• error message if input stream needs decompression
• warning message if input stream doesn't start with expected ##fileformat
• htslib 1.9