* Fix gh-issue #3:
* Add json_post.onerror.response INI entry, specifying whether and which
response code to send when `json_decode` fails.
* Add json_post.onerror.exit INI entry, specifying whether to exit PHP
without running the script when `json_decode` fails.
* Add json_post.onerror.warning INI entry, specifying whether to raise
a WARNING when `json_decode` fails.