Add supports the parse of COMMENT statements

[shinichi-takii]

Added

• Add supports the parse of COMMENT statements.
• Add DdlParseColumn.comment property.
• Add DdlParseColumn.description property for the DdlParseColumn.comment property alias.
• Add supports column comments in BigQuery DDL.

Applicable Issues

• fix If the ddl comment has multibyte characters, it will not be parsed correctly #35 - If the ddl comment has multibyte characters, it will not be parsed correctly
• fix Add supports column comments in BigQuery DDL #37 - Add supports column comments in BigQuery DDL
• fix Add supports the parse of COMMENT statements #38 - Add supports the parse of COMMENT statements

[coveralls]

Coverage remained the same at 100.0% when pulling 8c9a092 on feature/add-supports-comment-statements into 7328656 on master.