Configurable index filename

[mattgodbolt]

Currently the index for foo.gz is always foo.gz.zindex. Should be configurable, so it can live elsewhere. Maybe also the query should be able to cope with being passed the zindex file instead? Then, how to find the uncompressed data? Maybe store the path in the zindex?

[mattgodbolt]

Rephrasing this (As we now support configurable zindex file): use the full path stored in the zindex file to open the original file, and support zq-ing on the index file directly