Implements Custom User Configuration

This implements the ability to store user-specific configuration variables in `config.yaml`, which is not source controlled.  This solves the problem of having to manually edit `Vagrantfile` and risk accidental inclusion of changes back to the repository.

In the case that a `config.yaml` file does not exist, one is created based on the `config.yaml.dist` distributable configuration schema.  

Additional configuration options may be added over time.

- [x] Implementation
- [x] Add Documentation