Load features constants when creating NGLElement class (and subclass)
Load features constants when creating NGLElement class (and subclass)

This allows us to only load the feature constants once (per NLGElement
class and subclass instanciation), and simplify maintenance for the
__getattr__ method, allowing us to use w.plural (for example) to access
w.features['plural'] if it exists, or w._features['plural'], because
'plural' is a defined feature constant.

roman-kutlak/pynlg,brouberol/pynlg,mapado/pynlg