jsonfmt: Add pretty option

That completes the json_format() function for now. At least, it now does
everything I had planned for it.

Ended up at a bit over 300 LOC. That's larger than I had expected, but
still alright.

t/json_format.t

@@ -68,7 +68,7 @@ my @errors = (
     do { my $o = {}; bless $o, 'MyToJSONSelf' }, qr/MyToJSONSelf::TO_JSON method returned same object as was passed instead of a new one/,
 );
 
-plan tests => @tests*2 + @errors/2 + 9;
+plan tests => @tests*2 + @errors/2 + 10;
 
 for my($in, $exp) (@tests) {
     my $out = json_format $in;
@@ -90,6 +90,28 @@ for my ($in, $exp) (@errors) {
 is json_format({qw/a 1 b 2 c 3 d 4 d1 5 d11 6/, do { use utf8; qw/ü 7 月 8 💩 9/ }}, canonical => 1),
    do { use utf8; '{"a":"1","b":"2","c":"3","d":"4","d1":"5","d11":"6","ü":"7","月":"8","💩":"9"}' };
 
+is json_format(
+      { a => [], b => {}, c => { x => 1 }, d => { y => true, z => false }, e => [1,2,3] },
+      canonical => 1, pretty => 1
+    ), <<_;
+{
+   "a" : [],
+   "b" : {},
+   "c" : {
+      "x" : 1
+   },
+   "d" : {
+      "y" : true,
+      "z" : false
+   },
+   "e" : [
+      1,
+      2,
+      3
+   ]
+}
+_
+
 
 eval { json_format [[]], max_depth => 2 };
 like $@, qr/max_depth exceeded while formatting JSON/;