vcfnorm: when merging records, average QUALs and accumulate IDs and FILTERs

- when merging records with `norm -m+`, IDs were getting removed. This
  fixes that.
- FILTER was being taken from the first record seen. This would make it
  so that if any records FAIL, the site FAILs. Could add option to say
  that if any record PASSes, then PASS the entire site
- average the QUALs over records
Merge branch 'develop' of git://github.com/samtools/bcftools into feature/norm_merge_ids
Merge branch 'develop' of git://github.com/samtools/bcftools into feature/norm_merge_ids
vcfnorm: when merging, take max QUAL and add option for how to merge FILTER field

* now take the maximum QUAL value when merging
* option `strict-filter` will PASS record only if all records being merged PASS
  By default, of any site is PASS, merged record will PASS
Merge branch 'develop' of git://github.com/samtools/bcftools into feature/norm_merge_ids
Merge branch 'develop' of git://github.com/samtools/bcftools into feature/norm_merge_ids
Merge branch 'develop' of git://github.com/samtools/bcftools into feature/norm_merge_ids
vcfnorm: include docs and test for new `-s` option
Merge pull request #106 from mcshane/feature/norm_merge_ids

vcfnorm: merge ID, QUAL and FILTER