Add `build-config` command

[Sharpie]

It would be nice to have a Vagrant sub-command, vagrant build-config, that would load configuration, apply filters, and then output the results. Some feature ideas:

• Output the entire data structure, or a specific list of VMs.
• Output config_builder YAML, but make the generation step pluggable so that other export formats, such as Vagrantfiles, could be implemented.