add install_nbextension at top-level IPython.html

IPython/html/__init__.py

@@ -5,3 +5,5 b' import os'
 DEFAULT_STATIC_FILES_PATH = os.path.join(os.path.dirname(__file__), "static")
 
 del os
+
+from .nbextensions import install_nbextension
  No newline at end of file