Add sql_keywords option (fixes #384)

[watoo-tech]

You may pass a dict with a list of specific dialect keywords to consider.

Default behavior is to consider all supported dialects (oracle, plpgsql, hql).

keywords.keywords_map(*dialects) helper is provided to merge individual dialect maps.

You may use this to restrict keyword matching to a specific subset of dialects i.e. keywords_map(KEYWORDS_HQL,KEYWORDS_PLPGSQL)

See test_regression.py:test_issue384()

[watoo-tech]

This replaces PR #500 with a new one using its own branch instead of master.

[ajbosco]

@andialbrecht - Is there anything else this PR needs to be merged? We'd like to use sqlparse for Snowflake and this would be really helpful for adding the additional dialect keywords.

[ericxiao251]

What's the status on this PR? I would love to use this library to parse our BigQuery SQL.

Is there anything I can do to help? @andialbrecht