printer FEATURE allow printing unformatted XML

Optionally, allow printing XML without indentantion.
diff --git a/src/tree_data.h b/src/tree_data.h
index 594a151..25aabf1 100644
--- a/src/tree_data.h
+++ b/src/tree_data.h
@@ -42,6 +42,7 @@
 typedef enum {
     LYD_UNKNOWN,         /**< unknown format, used as return value in case of error */
     LYD_XML,             /**< XML format of the instance data */
+    LYD_XML_FORMAT,      /**< For input data, it is interchangeable with #LYD_XML, for output it formats XML with indentantion */
     LYD_JSON,            /**< JSON format of the instance data */
 } LYD_FORMAT;