Cobra upgrade: includes shell completion work (neilotoole#81)

Addressed neilotoole#80

Xlsx import handle empty sheets (neilotoole#79)

* can now import XLSX with empty sheets

* renamed XLSX test article

flag usage update

goreleaser update: artifacts no longer have version numbers

docs & misc

made sq repo public, at long last (neilotoole#75)

sq tbl truncate for postgres now outputs the number of rows truncated

fixed some panics with oddly-shaped xlsx imports (neilotoole#71)

README update

Initial work on a JSON driver (neilotoole#70)

* implementation work for json importers

* json driver checkpoint

* working on json.ParseObjectsInArray

* json.ParseObjectsInArray seems to be working

* checkpoint while tidying up ParseObjectsInArray

* more tidy checkpoint

* more tidy checkpoint 2

* tidying up ParseObjectsInArray

* tidy up

* code/docs cleanup

* more cleanup of json driver

* more cleanup of json driver

* flat json import seemingly working

* improvements to json driver

* json writer now prints empty [] for postgres empty tables