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/module3.yang b/tools/lint/examples/module3.yang
new file mode 100644
index 0000000..63754b1
--- /dev/null
+++ b/tools/lint/examples/module3.yang
@@ -0,0 +1,8 @@
+module module3 {
+  namespace "urn:yanglint:module";
+  prefix m;
+  leaf m { type string; must "../c/a"; }
+  container c {
+    leaf b { type string; }
+  }
+}