Add deprecated helper

Uses warning to show DeprecationWarning and custom stack trace

spacy/util.py

@@ -11,6 +11,8 @@ import sys
 import textwrap
 import random
 from collections import OrderedDict
+import inspect
+import warnings
 from thinc.neural._classes.model import Model
 import functools
 
@@ -501,6 +503,18 @@ def from_disk(path, readers, exclude):
     return path
 
 
+def deprecated(message, filter='always'):
+    """Show a deprecation warning.
+
+    message (unicode): The message to display.
+    filter (unicode): Filter value.
+    """
+    stack = inspect.stack()[-1]
+    with warnings.catch_warnings():
+        warnings.simplefilter(filter, DeprecationWarning)
+        warnings.warn_explicit(message, DeprecationWarning, stack[1], stack[2])
+
+
 def print_table(data, title=None):
     """Print data in table format.