add basic support for rails projects

Here's a stab at it. Seems to work fine for model specs, but when run on controller specs it fails the `noop` mutation.

For models:

```sh
mutant -r ./config/application.rb -r ./app/models/user.rb --rspec-rails ::User
```

For controllers:

```sh
mutant -r ./config/application.rb -r ./app/controllers/application_controller.rb -r ./app/controllers/users_controller.rb --rspec-rails ::UsersController
```