search: add preview and JSON options

[jqnatividad]

Primarily for qsv pro, add a --preview-matches <arg> option.

This will return the first <arg> matching rows to stderr (which is unbuffered, and will immediately flush) while the rest of the input CSV is being scanned.

This will allow instantaneous return of the first N matches regardless of the size of the input file, as search will continue for the rest of the file.

The --json option supports the return the results in JSON format.

cc @rzmk @samibaig

[jqnatividad]

Thinking about this more, the --preview-matches <arg> with having a double meaning, either as time elapsed (in ms) or number of rows, whichever comes first (default: 100).

We choose 100 as it seems to be the shortest perceivable application response delay per https://stackoverflow.com/questions/536300/what-is-the-shortest-perceivable-application-response-delay