Add the describe command

...which should, hopefully, show useful information about a particular
YANG node, such as the YANG-level description, the units, type info,
etc.

Change-Id: Id430ae58fe353124e5132fa5a69378e98932ebce
diff --git a/src/ast_handlers.hpp b/src/ast_handlers.hpp
index 456ef11..a8dd4ae 100644
--- a/src/ast_handlers.hpp
+++ b/src/ast_handlers.hpp
@@ -497,6 +497,8 @@
 
 struct commit_class;
 
+struct describe_class;
+
 struct help_class;
 
 struct get_class;