Make arguments for sqlite3.connect accessible

Added an optional keyword argument `dbargs` which are passed through to the connect method of the SQLite 3 driver. Now it's possible e.g. to use the `detect_types` parameter for automatically parsing timestamps to datetime objects.

```python
sqlite.Plugin(dbfile='/path/to/database.db', dbargs={'detect_types': sqlite3.PARSE_DECLTYPES})
```