✨ implemented an indentation character #520

An optional parameter for dump() allows to set the character to use for
indentation (default: space). In case a JSON value is serialized to an
output stream, its fill character is used (and can be set with
std::setfill).

doc/examples/operator_serialize.output

@@ -15,3 +15,8 @@
   16
 ]
 
+{
+	"one": 1,
+	"two": 2
+}
+