Support no parser fulltext index

No parser fulltext index is useful for predictive search by "XXX*"
groonga query syntax. You can create no parser fulltext index by the
following SQL:

  CREATE TABLE table (
    -- ...
    FULLTEXT INDEX (term) COMMENT 'parser "off"'
  ) ENGINE=mroonga;

"off" is compared with case insensitive. We use "off" because "SHOW
VARIABLES" uses "on" and "off" for boolean value.

[groonga-dev,01307]