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-reply.xml b/tools/lint/examples/action-reply.xml
new file mode 100644
index 0000000..5a4788e
--- /dev/null
+++ b/tools/lint/examples/action-reply.xml
@@ -0,0 +1 @@
+<leaf3 xmlns="urn:module4">some_output</leaf3>