Add file completion on rspec command

When I use rspec-console, I need spec file name completion, because there are no file name completion in rails pry console.

This works as...

```
❯ bundle exec rails c
Loading development environment (Rails 4.1.0)
[1] pry(main)> rspec spec/models/[TAB]
```

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This is totally off the topic, but I want say thank you for this nice gem. My favorite. :gem: