Add an option to filter rows

[bartholomew-gander]

It would be good to add a command line option for a filter expression to limit what rows are exported. Perhaps something like:

--filter-expression "Artist = :v1" --expression-attribute-values file://expression-attributes.json

as described here in the documentation: https://docs.aws.amazon.com/cli/latest/reference/dynamodb/scan.html

I'm willing to work on that feature