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.
Chef is set to force the the file's content to be equal to what I've set. The mechanism doesn't really matter. (Though behind the scenes, it makes a temporary file in /var/chef/cache, creates md5sums of both the original file and the new file, and then moves the new file on top of the old file. It also shows a diff output of the new file in debug level output.)