new options to print stats based on the index files

* add options to index to print stats from CSI or TBI index file
* `--stats` outputs contigs and num records per-contig
* `--nrecords` outputs total number of records
* tests and man docs
update index stats option to output contig lengths too

* contig length printed as "." if default length 2147483647 found
Merge pull request #104 from mcshane/feature/idxstats-v2

idxstats in bcftools index