Add callback interface for errors

Printing to stderr is not the right answer for a library.
Make length return abs value of numeric inputs
Add index strings by string; return string indexes

    % jq '.[","]'
    "a,bc,def,ghij,klmno"
    [1,4,8,13]
    %
Add index and rindex builtins
Add tests for string index by string and builtins
Add self to AUTHORS
Add jv_parser_new() flags for parsing options
Add -t/-i/-k tolerate/ignore/keep-malformed-inputs
Document malformed JSON text options
Use AC_CHECK_FUNCS(), define _GNU_SOURCE

Check for these extensions:

 - asprintf()
 - vasprintf()
 - memmem()