Implement NextToken/MaxRecords for ListShards

[slydon]

Hello! I was attempting to use this library to test some implementation issues with ListShards in Kinesis client libraries (including aws-cli). I was seeing that MaxResults wasn't being used and no NextToken implementation was functioning other than throwing the error I expect when the StreamName is provided.

I see: https://github.com/mhart/kinesalite/blob/master/actions/listShards.js#L12

Would this be hard to implement? This language isn't really my wheelhouse and I don't see a NextToken implementation in here or in dynalite.