Dynamic environment updates

Parse `.env.example` (in a simple fashion) and create `.env` based on the merge of the default values and any user-specified ones.

You also [previously asked about disabling provisioning](https://github.com/alias1/huginn-chef/pull/1#discussion_r17517264). Vagrant supports disabling provisioning via the `--no-provision` command line flag (opt-out behavior) which I find more convenient. We could switch to an opt-in behavior, but I think provisioning by default is the expected behavior. Either way, we should document the additional behavior in the README.
Can you add this part back in after all your automagic dotenv file writing stuff. It's needed to make the foreman export work properly/etc