Allow to use custom options

[makaivelli]

It would be handy to allow using custom options when constructing a DynamoDB object.
For start it would be nice to support defining convertEmptyValues and port in serverless.yml.

[bart]

As far as I can see you can simply set it after you have initialized dynamodb like this:

const dynamoDb = require('serverless-dynamodb-client').doc
dynamoDb.options.convertEmptyValues = true