pgsql: Add word_similarity to pg_trgm contrib module.

Add word_similarity to pg_trgm contrib module.

Patch introduces a concept of similarity over string and just a word from
another string.

Version of extension is not changed because 1.2 was already introduced in 9.6
release cycle, so, there wasn't a public version.

Author: Alexander Korotkov, Artur Zakirov

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/f576b17cd6ba653bdace1f0da9a3b57f4984e460

Modified Files
--------------
contrib/pg_trgm/Makefile | 2 +-
contrib/pg_trgm/expected/pg_trgm.out | 4 +-
contrib/pg_trgm/pg_trgm--1.1--1.2.sql | 64 ++++-
contrib/pg_trgm/pg_trgm--1.2.sql | 64 +++++
contrib/pg_trgm/sql/pg_trgm.sql | 4 +-
contrib/pg_trgm/trgm.h | 30 +-
contrib/pg_trgm/trgm_gin.c | 20 +-
contrib/pg_trgm/trgm_gist.c | 33 ++-
contrib/pg_trgm/trgm_op.c | 501 ++++++++++++++++++++++++++++++----
doc/src/sgml/pgtrgm.sgml | 79 ++++++
10 files changed, 726 insertions(+), 75 deletions(-)