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/notification.xml b/tools/lint/examples/notification.xml
new file mode 100644
index 0000000..803ddad
--- /dev/null
+++ b/tools/lint/examples/notification.xml
@@ -0,0 +1,3 @@
+<notif2 xmlns="urn:module4">
+  <leaf7/>
+</notif2>