Support enhanced plugins object structure

[vkartaviy]

This plugin doesn't support enhanced plugins object throwing below error

TypeError: _.get(...).includes is not a function
    at ServerlessDynamodbLocal.hasAdditionalStacksPlugin (~/node_modules/serverless-dynamodb-local/index.js:249:51)

The serverless plugins section supports two kind of formats:

Array object:

plugins:
  - plugin1
  - plugin2

Enhanced plugins object:

plugins:
  localPath: './custom_serverless_plugins'
  modules:
    - plugin1
    - plugin2

This PR add support for both of them.

[AshanFernando]

Thanks @vkartaviy . I have approved the changes. Waiting for a second review and approval before merging. There is another PR for the same purpose, sent before this. Will you be able to review that and see it serves the same purpose. If so, I could go ahead with that one. #214

[vkartaviy]

@AshanFernando parsePluginsObject was introduced in serverless v1.28.0 so my PR may have compatibility issues with older serverless releases.

[cyberwombat]

What's the status with this?