* Extend count_by method

spacy/tokens/doc.pyx

@@ -218,7 +218,7 @@ cdef class Doc:
                 output[i, j] = get_token_attr(&self.data[i], feature)
         return output
 
-    def count_by(self, attr_id_t attr_id, exclude=None):
+    def count_by(self, attr_id_t attr_id, exclude=None, PreshCounter counts=None):
         """Produce a dict of {attribute (int): count (ints)} frequencies, keyed
         by the values of the given attribute ID.
 
@@ -237,13 +237,23 @@ cdef class Doc:
         cdef attr_t attr
         cdef size_t count
         
-        cdef PreshCounter counts = PreshCounter(2 ** 8)
-        for i in range(self.length):
-            if exclude is not None and exclude(self[i]):
-                continue
-            attr = get_token_attr(&self.data[i], attr_id)
-            counts.inc(attr, 1)
-        return dict(counts)
+        if counts is None:
+            counts = PreshCounter(self.length)
+            output_dict = True
+        else:
+            output_dict = False
+        # Take this check out of the loop, for a bit of extra speed
+        if exclude is None:
+            for i in range(self.length):
+                attr = get_token_attr(&self.data[i], attr_id)
+                counts.inc(attr, 1)
+        else:
+            for i in range(self.length):
+                if not exclude(self[i]):
+                    attr = get_token_attr(&self.data[i], attr_id)
+                    counts.inc(attr, 1)
+        if output_dict:
+            return dict(counts)
 
     def _realloc(self, new_size):
         self.max_length = new_size