extensions CHANGE metadata substatements

Support processing of all the Metadata annotation's substatements. Also
add basic test for overall annotation implementation
diff --git a/Doxyfile.in b/Doxyfile.in
index dbc9674..e47a7ae 100644
--- a/Doxyfile.in
+++ b/Doxyfile.in
@@ -784,6 +784,7 @@
 INPUT                  = ./src/libyang.h \
 			 ./src/context.h \
                          ./src/tree_schema.h \
+                         ./src/plugins_exts.h \
                          ./src/log.h \
                          ./src/set.h \
                          ./src/dict.h