Allow custom install path

[aneilbaboo]

Related to: 99x/dynamodb-localhost#22

Requires: 99x/dynamodb-localhost#24

Changes: This implements changes proposed in 99x/dynamodb-localhost#22

Demo Link: I created an npm fork which demonstrates the features working:

# in serverless.yml:
plugins:
  - "@aneilbaboo/serverless-dynamodb-local"

# install the plugin, then install dynamo:
npm install @aneilbaboo/serverless-dynamodb-local
serverless dynamodb install # installs to <project-root>/.dynamodb by default

[mjzone]

Thanks again @aneilbaboo 👍