Duckql

- **Feature**: Introduced `functions.StringAgg`
- **Feature**: Introduced `structures.CastOperator`
- **Feature**: CLI for parsing files

- **Change**: Allow direct use of `structures.Comparision` in query conditions

- **Fix**: Allow aliases in `structures.Query`
- **Change**: Do not escape `%`

- **Feature**: Natural join support

- **Change**: Completed docs
- **Feature**: Recursive nested properties lookup in `Query` object

- **Change**: Default values for optional attributes in `structures.Query`
- **Change**: `custom_parser` now can raise `ParseError` if there is invalid object type passed