Change tabix index type from TBI to CSI, allow chromosome size > 512 Mb

[zoomlion]

Change tabix index type from TBI to CSI, allow chromosome size > 512 Mb

Closes #112

[zqbake]

Could placing '-C' after the filename cause tabix to fail (options usually go before the file)? Also, is there a minimum tabix/htslib version needed for '-C' — could this error out on older systems?
Do we want to force .csi or make the index type configurable (auto/csi/tbi)? Forcing .csi might break workflows/tools expecting .tbi.

[Nanguage]

Consider still being compatible with .tbi, for example, detecting if a TBI index already exists so that a .tbi index does not need to be created. Additionally, add detection for the htslib/tabix version, and automatically downgrade to using tbi if the requirements are not met.