Add basic `vagrant push` skeleton

- [x] Create skeleton
- [ ] Wire `vagrant push` to invoke the plugin
  - [ ] `vagrant push` with no strategy when there is one strategy in the Vagrantfile pushes
  - [ ] `vagrant push` with no strategy when there are multiple strategies in the Vagrantfile errors
  - [ ] `vagrant push` with no strategy when there are no strategies in the Vagrantfile errors
- [ ] Hook up the config so `config.vm.push` yields the proper config structure