Support using existing table as lexicon

Comment content in the following "FULTEXT INDEX" line is the point:

    CREATE TABLE Tags (
      name VARCHAR(64) PRIMARY KEY
    ) DEFAULT CHARSET=utf8
      COLLATE=utf8_bin
      COMMENT='default_tokenizer "TokenDelimit"';

    CREATE TABLE Bugs (
      id INT UNSIGNED PRIMARY KEY,
      tags TEXT COMMENT 'flags "COLUMN_VECTOR", type "Tags"',
      FULLTEXT INDEX tags_index (tags) COMMENT 'table "Tags"'
    ) DEFAULT CHARSET=utf8;

This generates the following groonga schema:

    table_create  Tags TABLE_PAT_KEY ShortText --default_tokenizer TokenDelimit
    column_create Tags name COLUMN_SCALAR ShortText

    table_create  Bugs TABLE_PAT_KEY UInt32
    column_create Bugs id COLUMN_SCALAR UInt32
    column_create Bugs tags COLUMN_VECTOR Tags

    column_create Tags index COLUMN_INDEX|WITH_POSITION Bugs tags

Index for Bugs.tags is defined in Tags.

TODO: Use index name specified by FULLTEXT INDEX instead of
"index". We cannot create two or more indexes.