Add clear-output as a convenience flag per @rjleveque's in-person suggestion

[mpacer]

Talking with @fperez and @rjleveque today pointed out that nbconvert should be able to apply the ClearOutputPreprocessor easily on a file. However doing so is unnecessarily verbose, e.g.:

jupyter nbconvert --ClearOutputPreprocessor.enabled=True --inplace example.ipynb

This creates a flag that makes this much more concise.

jupyter nbconvert --clear-output example.ipynb

[mpacer]

Does anyone have an objection to this?

[takluyver]

Looks good to me. Don't forget to mention it in the release notes :-)