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/action.xml b/tools/lint/examples/action.xml
new file mode 100644
index 0000000..b0bd411
--- /dev/null
+++ b/tools/lint/examples/action.xml
@@ -0,0 +1,10 @@
+<action xmlns="urn:ietf:params:xml:ns:yang:1">
+  <cont1 xmlns="urn:module4">
+    <list>
+      <leaf1>key_val</leaf1>
+      <act>
+        <leaf2>some_input</leaf2>
+      </act>
+    </list>
+  </cont1>
+</action>