allow IPythonWidget.in_prompt to exclude input number

This allows common prompts like '>>> ' in the QtConsole, and prevents
failures of string formatting from crashing the console.

IPython/frontend/qt/console/ipython_widget.py

@@ -440,7 +440,11 b' class IPythonWidget(FrontendWidget):'
     def _make_in_prompt(self, number):
         """ Given a prompt number, returns an HTML In prompt.
         """
-        body = self.in_prompt % number
+        try:
+            body = self.in_prompt % number
+        except TypeError:
+            # allow in_prompt to leave out number, e.g. '>>> '
+            body = self.in_prompt
         return '<span class="in-prompt">%s</span>' % body
 
     def _make_continuation_prompt(self, prompt):