Fix argument parsing for Twitter bots.

Even though we support a command-line option of --user=,
it gets stored in a field called 'email'.

(imported from commit f2956524517a93187ed182caf8e2d85ccbc1a0f4)

demos/twitter-search-bot

@@ -119,7 +119,7 @@ Please double-check and try again."
     sys.exit()
 
 client = zulip.Client(
-    email=opts.user,
+    email=opts.email,
     api_key=opts.api_key,
     site=opts.site,
     verbose=True)