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/module1.yang b/tools/lint/examples/module1.yang
new file mode 100644
index 0000000..1df7bf1
--- /dev/null
+++ b/tools/lint/examples/module1.yang
@@ -0,0 +1,5 @@
+module module1 {
+  namespace "urn:yanglint:module";
+  prefix m;
+  leaf m { type string; }
+}