ENH: change the clean --before shortopt from -b to -e

docs/source/whatsnew/1.0.3.txt

@@ -56,4 +56,6 @@ None
 Miscellaneous
 ~~~~~~~~~~~~~
 
-None
+* Changed the short-opt for ``--before`` in the ``zipline clean``
+  entrypoint. The new argument is ``-e``. The old argument, ``-b``, conflicted
+  with the ``--bundle`` short-opt (:issue:`1625`).

zipline/__main__.py

@@ -323,7 +323,7 @@ def ingest(bundle, assets_version, show_progress):
     help='The data bundle to clean.',
 )
 @click.option(
-    '-b',
+    '-e',
     '--before',
     type=Timestamp(),
     help='Clear all data before TIMESTAMP.'