make ProvisionedThroughput configurable

[ianhuynh]

would you accept a PR to allow ProvisionedThroughput to be configurable?

e.g. 
var options = {
    // Optional DynamoDB table name, defaults to 'sessions'
    table: 'myapp-sessions',

    // Optional path to AWS credentials and configuration file
    // AWSConfigPath: './path/to/credentials.json',

    // Optional JSON object of AWS credentials and configuration
    AWSConfigJSON: {
        accessKeyId: <YOUR_ACCESS_KEY_ID>,
        secretAccessKey: <YOUR_SECRET_ACCESS_KEY>,
        region: 'us-east-1'
    },

    // Optional client for alternate endpoint, such as DynamoDB Local
    client: new AWS.DynamoDB({ endpoint: new AWS.Endpoint('http://localhost:8000')}),

    // Optional clean up interval, defaults to 600000 (10 minutes)
    reapInterval: 86400000,    // 1 day

   readCapacityUnits: 20,
   writeCapacityUnits: 20 
};

[ca98am79]

yes, thank you

[ca98am79]

implemented with #40