Load a specific rbenv-vars file

This let's you load a specific rbenv-vars file:

```bash
$ RBENV_VARS=rbenv_vars.test bundle exec rake
```
 
e.g. in Rails you would typically have several of these files in your project: each with environment- (or stage-) specific variables. Switching environments is easier this way.

The file is appended to the files rbenv-vars considers and is 'evaluated' last.