yanglint FEATURE initial version of yanglint(1)

Source codes are mostly taken from libyang 1.0 and slightly modified for
libyang 2.0. A lot of functionality is still missing since it is not yet
provided by libyang 2.0
diff --git a/tools/lint/examples/nested-notification.xml b/tools/lint/examples/nested-notification.xml
new file mode 100644
index 0000000..024b65a
--- /dev/null
+++ b/tools/lint/examples/nested-notification.xml
@@ -0,0 +1,8 @@
+<cont1 xmlns="urn:module4">
+  <list>
+    <leaf1>key_val</leaf1>
+    <notif1>
+      <leaf4>some_value</leaf4>
+    </notif1>
+  </list>
+</cont1>