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.
I assume you're talking about just the RBENV_VERSION line. I'll add it back.