Fix for issue #140, add vcf_record_sort_key arg

- Added 'vcf_record_sort_key' which allows users to specify any chromosome ordering.
- Fixed issue #140 by emitting all records from the current chromosome before moving on to the next one. This takes care of issue #140 in the typical case (when all files have records for all contigs), but still fails in some edge cases (when vcf files have differing sets of contigs and where the alphabetical order of these contigs is not the desired order). In those edge cases, the user can set the 'vcf_record_sort_key' arg to explicitly defining the chromosome order - in which case the issue would be completely solved.