Added user defined attribute declarations.

This is such that if you mis-spell an attribute, it doesn't get
silently ignored.

Bug: 18294628
Change-Id: I10013f5b2a21048b7daba2e9410678f528e09761
Tested: on Linux.

tests/monster_test.fbs

@@ -4,6 +4,8 @@ include "include_test1.fbs";
 
 namespace MyGame.Example;
 
+attribute "priority";
+
 enum Color:byte (bit_flags) { Red = 0, Green, Blue = 3, }
 
 union Any { Monster }  // TODO: add more elements