added --autoreload flag to NotebookApp (#4795)

* added `--autoreload` flag

When passed, the webapp will watch for any changes to its Python source. On
change, all changed packages will be reimported and the webapp will restart.
Also works on Python source files in Jupyter server extensions. Implemented
using the built in `autoreload` parameter in the constructor of
`tornado.web.Application`.

* updated .gitignore

Author: telamonian

.gitignore

@@ -42,3 +42,12 @@ config.rst
 package-lock.json
 geckodriver.log
 *.iml
+
+# jetbrains IDE stuff
+*.iml
+.idea/
+
+# ms IDE stuff
+*.code-workspace
+.history
+.vscode