Partially apply doc/span/token into method

We want methods to act like they're "bound" to the object, so that you can make your method conditional on the `doc`, `span` or `token` instance --- like, well, a method. We therefore partially apply the function, which works like this:

```
def partial(unbound_method, constant_arg):
    def bound_method(*args, **kwargs):
        return unbound_method(constant_arg, *args, **kwargs)
    return bound_method

spacy/tokens/underscore.py

@@ -1,3 +1,5 @@
+import functools
+
 class Underscore(object):
     doc_extensions = {}
     span_extensions = {}
@@ -22,7 +24,7 @@ class Underscore(object):
         if getter is not None:
             return getter(self._obj)
         elif method is not None:
-            return method
+            return functools.partial(method, self._obj)
         else:
             return self._doc.user_data.get(self._get_key(name), default)