[WIP] Private networks

I had a need for private networks last week and this is my first stab at implementing GH-120

### TODO

- [x] Integrate pipework into the plugin
- [x] Remove docker specifics from the script
- [x] Make it work with a single ip range (`192.168.1.X`)
- [ ] Clean up on `vagrant halt` (use `brctl show brX` to check if in use and `ip link delete brX` to clean up)
- [ ] Make it work for any ip range
- [ ] Make sure it works when bringing up containers in parallel
- [ ] Improve logging
- [ ] Update docs