Make :project_path configurable

Sometimes you might want to run `librarian-chef` at a deeper level than your `Vagrantfile`. For instance, if you're building a project with multi-vendor support:

```
.
├── Vagrantfile
├── chef
|   ├── Cheffile
|   └── cookbooks
├── puppet
|   ├── Puppetfile
|   └── modules
└── tests
```

Fairly certain it involves this line in the librarian framework and `vagrant-librarian-chef`:
- https://github.com/applicationsonline/librarian/blob/master/lib/librarian/environment.rb#L30
- https://github.com/jimmycuadra/vagrant-librarian-chef/blob/master/lib/vagrant-librarian-chef/action/librarian_chef.rb#L16

PR pending