add support for unspecified type/table field
this is to support syntax as following:
```
!table Table:
Unspecified
```
@@ -55,6 +55,7 @@ table -> mode=("!table"|"!type") NAME attribs? ":"
table_row -> table_field
| annotation
| table
+ | unspecified=TEXT_WITH_WHITESPACE
| SHORTCUT;
table_field -> field_with_default_value optional="?"? attribs? (":" annotations)?;
