'Add a new function to extract features from a document. This function will
be used in extract_features().
Important: in this step our kwargs are only representing additional parameters,
and NOT the document we have to parse. The document will always be the first
parameter in the parameter list, and it will be added in the extract_features()
function.
:param function: the extractor function to add to the list of feature extractors.
:param kwargs: additional parameters required by the `function` function.'

def add_feat_extractor(self, function, **kwargs):

    self.feat_extractors[function].append(kwargs)
