Implement stemmers

[piki]

This implements the stemmer feature from TIN v1.0.4. Remember to DROP EXTENSION tin CASCASDE ; CREATE EXTENSION tin, or at least update the extension and rebuild all your full-text indexes.

tin=# CREATE INDEX docs_body_tin ON docs USING tin (body) WITH (stemmer = 'en');
CREATE INDEX
tin=# select * from docs;
 id |  body
----+---------
  1 | mold
  2 | moldy
  3 | Molding
  4 | beer
  5 | run
  6 | runs
  7 | Running
(7 rows)

tin=# select * from docs where body ==> 'molDy';
 id | body
----+-------
  2 | moldy
(1 row)

tin=# select * from docs where body ==> 'molded';
 id |  body
----+---------
  1 | mold
  3 | Molding

Closes #16