Add support for case statements

Add highlight for statements that look like the following (supported by
Transact-SQL and PL/SQL, among others):

CASE [ expression ]
   WHEN condition_1 THEN result_1
   WHEN condition_2 THEN result_2
   ...
   WHEN condition_n THEN result_n
   ELSE result
END
Merge pull request #5 from abahgat/case-support

Add support for case statements