Add "collection" method

Add Rails routing style convenience method for defining endpoints, eg:

```ruby
class Project < Rapidash::Base
  collection :archived
end

# creates the method below which performs a
# GET /projects/archived
client.projects.archived!

class Project < Rapidash::Base
  collection :delete_all, path: 'destroy', method: :post
end

# creates the method below which performs a
# POST /projects/destroy
client.projects.delete_all!
```

- [x] Write specs
- [x] Documentation