Automatic export env before provision

Added possibility to automatically export variables on provision to env file.
This can be used in Vagrant by doing `config.user.auto_export = :format`
It's not necessary to keep an instance variable for `@machine`, `@global_env` and `@export_format`. For the first two, they are only used to fetch `@config` more easily. I would define a method to do this job something like `extract_config(env)`.

For the second (`@export_format`), I would use `@config.user.auto_export` directly instead.