Pass the encoding from the config to DataSource

I've run into a small issue where the `DataSource` object was defaulting `encoding` (to UTF-8) on some census files (they're latin-1, so issues don't pop up often).

This patch passes the config object into the code that creates the `DataSource` objects, so that the encoding gets passed down into the `OFTString` by way of the feature encoding (which pulls from the `DataSource`)

Happy to fix it a different way if this has any issues I missed.

Thanks!