Add consume spaces to grammar parser.

This lets us handle fragments such as:

```
TOKEN  :   DEFINITION
```

where there is space before the colon in the rule.

Author: pmfirestone

src/grammar.rs

@@ -199,6 +199,8 @@ impl EBNFParser {
             self.parse_priority();
         }
 
+        self.consume_space();
+
         if self.peek(0) == Some(':') {
             self.consume(1);
         } else {
@@ -230,6 +232,8 @@ impl EBNFParser {
             self.parse_priority();
         }
 
+        self.consume_space();
+
         if self.peek(0) == Some(':') {
             self.consume(1);
         } else {