Add an option to check if dropping is not preferred

[ibiris]

https://github.com/better/jsonschema2db/blob/60832f5f38d6a257920508c09fec3a65daed4e4c/jsonschema2db.py#L238

In create_tables I should be able to direct the call to do nothing if the schema exists. Maybe add an option to direct create_tables to avoid dropping the schema.