Attribute grammar should allow trailing commas

The attribute list, and the interior list of values have grammars:

```
attribute-list:
  attribute
  attribute-list  ,  attribute

attribute-values
  attribute-value
  attribute-values  ,  attribute-value

```

Both of which imply that the lists may not end in a comma. We wish to allow non-empty lists to end in a comma in both cases.

