Jupyter notebook snippets menu

Adds a customizable menu item to Jupyter notebooks to insert snippets, boilerplate, and examples of code. This notebook extension adds a menu item (or multiple menu items, if desired) after the Help menu in Jupyter notebooks. This new menu contains little snippets of code that we all forget from time to time but don't want to google, or are just too lazy to type, or simply didn't know about. It can also be helpful for people just starting out with a programming language, who need some ideas for what to do next — like importing a module, defining variables, or calling functions. The new menu comes with a default value relevant for python programming — especially scientific computing — though this is fully user-configurable as detailed below. The default menu is named Snippets, and contains sub-menus with snippets for a few popular python packages, as well as python itself, and some notebook markdown.