Support for additional environment override on command line option.

Currently, Sails only support for two environment config files, development and production. This pull request would like to add additional environments e.g. staging, qa, etc.

Specifying environment via command line:

sails lift --environment=staging

Specifying the grunt task name to run for this environment:

env/staging.js
gruntTaskName: 'prod'   staging is reusing production grunt task.