add ON UPDATE clause to fk definition grammar rule (#18)
add ON UPDATE clause to fk definition grammar rule (#18)

Statements of the form
```
ALTER TABLE ONLY tablename ADD CONSTRAINT FOREIGN KEY fk_name
  REFERENCES foreign_table(col)
  [ DEFERRABLE ]
  [ ON UPDATE ( CASCADE | RESTRICT | NO ACTION | SET NULL | SET DEFAULT ) ]
  [ ON DELETE ( CASCADE | RESTRICT | NO ACTION | SET NULL | SET DEFAULT ) ];
```
are now also supported, and therefore foreign key definitions are not
dropped by the parser if they contain a ON UPDATE clause.

Co-authored-by: Max Franke <41281de880f39d071e2b275eba24e5f49a7425c6@vis.uni-stuttgart.de>
rm-hull/sql_graphviz