Smart preview selection (e.g. preview first n rows; latest rows; sample)

[jqnatividad]

• preview first n rows
• latest rows

[jqnatividad]

This is currently WIP and being implemented with the qsv download command.
dathere/qsv#654

[jqnatividad]

preview last n rows implemented in c5ada80

If PREVIEW_ROWS is negative, create preview slice from the end of the file

[jqnatividad]

negative PREVIEW_ROWS has been implemented in 0.8.0.

Closing this now and will create a dedicated issue for preview sampling.