More useful notebook-specific lints

[bluss]

flake8 assumes variables in global scope can be defined but never used. For notebooks, it would be useful to lint for variables that are unused and in particular functions that are never used.

Linting with flake8 is party useful (it flags errors you wouldn't see until restarting the kernel and rerunning, otherwise) and partly flags lots of things that don't matter in a notebook, like empty lines between functions. With some configuration it gets better.

There seems to be a whole class of errors we could detect in notebooks that aren't implemented in flake8 yet, though. Is there anything we can do in this extension (thanks for making it), and/or what do we need to go to flake8 to implement?