MINOR: sample: Added support for Arrays in sample_conv_json_query in sample.c

MINOR: sample: Added support for Arrays in sample_conv_json_query in sample.c

Method now returns the content of Json Arrays, if it is specified in
Json Path as String. The start and end character is a square bracket. Any
complex object in the array is returned as Json, so that you might get Arrays
of Array or objects. Only recommended for Arrays of simple types (e.g.,
String or int) which will be returned as CSV String. Also updated
documentation and fixed issue with parenthesis and other changes from
comments.

This patch was discussed in issue #2281.

Signed-off-by: William Lallemand <wlallemand@haproxy.com>