Add custom extension to paths

This feature was requested in #230.

This feature introduces the `collection_path_extension` setting. It will append the specified extension to all the routes/paths in that model

### Custom extension path

If you need to add an extension to your paths:

```ruby
class User
  include Her::Model
  collection_path_extension ".json"
end

users = User.all
# GET "/users.json"

user = User.find(1)
# GET "/users/1.json"
```

Please let me know if I should change anything.