Added new "manual" deploy option

This adds a new `deployType: 'manual'` option which uploads the deploy archive to S3, creates a new version on Elastic Beanstalk, but leaves final deployment step to be done "manually". 

I considered making this the default, since I almost made the mistake of deploying code to our live production environment while initially testing this plugin! :worried:

P.S. There is some code-style and documentation stuff in the 2nd commit. If you don't like these changes feel free to cherry-pick the 1st commit.