Allow use of docker container for npm install

[swhite24]

Related to #168
This PR does a couple of things:

1. Allow a docker image (-I, --dockerImage) to be provided to either the package or deploy command. This image will then be used as the environment in which npm install will be run. An example of the type of image you'd want to use is https:/lambci/docker-lambda .
   There is also an assumption made that the image matches the lambda environment, such as mounting the code directory to and setting the working directory to /var/task.
2. Changes the code directory to $PWD/.lambda. On a mac, os.tmpDir() resolves to a symlink, which are notoriously difficult to work with when mounting volumes inside a docker container. Changing the code directory seemed like a simple approach to alleviate those concerns, though I realize this is a somewhat inconvenient change.

Example usage:

node-lambda package -I alchemy/lambda:build -A pkg --handler dist/handler.handler  --functionName foo

[DeviaVir]

Works, and seems like a really clever idea. Thanks for contributing! 👍