Fix regression involving busybox awk (#17).

Several fixes for better RFC8259 compliance.
New option -v STRICT=1 to enforce character escape validation.
Fix JSON.awk always exists 0.
Fix gawk 5.1.0 warning.
Add comparative test results.
Tested with the JSONTestSuite.
Update documentation.

Improve compatibility with old awk versions (macOS, "onetrueawk").

Accept BOM marked files.

This release extends the callback interface that was added in version 1.2 with hooks into parser events and jpath construction. This allows to more easily write applications that transform JSON into other representations.
js-dot-path.awk, a callback file that transforms JSON into javascript, is included as an example of the new capabilities. opera-bookmarks.awk is another example.

It is now possible to selectively censor the following values from the output stream: "", [], {}.

Documentation was updated.

This release adds callbacks to allow embedding JSON.awk in other awk programs, and updates documentation. A minor issue involving output selection was also corrected.

Clarify options STREAM and BRIEF.
Fix and document stub function apply().