Pass all kwargs from pipeline to _pipeline

This a) errors out on invalid keywords, typos, etc. and b) makes it
easier to add additional keyword args to `_pipeline`; they just need to
be added in the `_pipeline` def.

Author: Lukas Schrangl

slicerator.py

@@ -511,15 +511,15 @@ def pipeline(func=None, **kwargs):
     >>> red_img = red_channel(single_img)  # normal behavior
     """
     def wrapper(f):
-        return _pipeline(f, kwargs.get("retain_doc", False))
+        return _pipeline(f, **kwargs)
 
     if func is None:
         return wrapper
     else:
         return wrapper(func)
 
 
-def _pipeline(func, retain_doc):
+def _pipeline(func, retain_doc=False):
     """Actual `pipeline` implementation
 
     Parameters