Support vi-mode 'InputFormat' prompt

Prompt defining what vi-mode is in, is either 'vi-insert' or 'vi-navigate' (or something 'vi-...' I imagine).
From this `[3:6]` is extracted to get either 'ins' or 'nav' in the prompt.
This is consistent with the behavior I experience when manipulating a custom prompt from the most updated pip installable package.

However, in the case of installing the current master from source, the prompt is on the format 'InputFormat.INSERT' and 'InputFormat.NAVIGATION'.

IPython/terminal/prompts.py

@@ -17,7 +17,12 b' class Prompts(object):'
     def vi_mode(self):
         if (getattr(self.shell.pt_app, 'editing_mode', None) == EditingMode.VI
                 and self.shell.prompt_includes_vi_mode):
-            return '['+str(self.shell.pt_app.app.vi_state.input_mode)[3:6]+'] '
+            mode = str(self.shell.pt_app.app.vi_state.input_mode)
+            if mode.startswith('InputMode.'):
+                mode = mode[10:13].lower()
+            elif mode.startswith('vi-'):
+                mode = mode[3:6]
+            return '['+mode+'] '
         return ''