Optionally decode DynamoDB sets to Elixir MapSets

When encoding Elixir MapSet data, the default behavior of this application is to convert that data to one of DynamoDB's three set types - number, string, or binary; however, DynamoDB sets will be decoded to Elixir List data.

This minor release adds a configuration option that allows a user to specify how to decode DynamoDB set types - the default will remain List, but setting decode_sets: true in the app's configuration will decode those set types to an Elixir MapSet.

This release also includes several other small bits of code cleanup and housekeeping, and adds additional test coverage.