Added getattr() support

CHANGES

@@ -1,3 +1,8 @@
+Version 0.4
+-----------
+9/5/2018   Added support for getattr() and related functions on
+           productions.
+
 Version 0.3
 -----------
 4/1/2018   Support for Lexer inheritance added.  For example:

sly/yacc.py

@@ -144,7 +144,10 @@ class YaccProduction:
         raise AttributeError('No index attribute found')
 
     def __getattr__(self, name):
-        return self._slice[self._namemap[name]].value
+        if name in self._namemap:
+            return self._slice[self._namemap[name]].value
+        else:
+            raise AttributeError(f'No symbol {name}')
 
     def __setattr__(self, name, value):
         if name[0:1] == '_' or name not in self._namemap: