Python Tutor: support showing cells without input

[DavidLeoni]

Current setup:

• Jupyter notebooks: fine
• website too verbose
• PDF: shows input but no viz
• slides: decent (although verbose) by implementing Preprocessor: support jupman-purge-text-output  #161

Desiderata:

• website: show no jupman.pytut call nor jupman-related instructions
• PDF: show input and as viz only latest execution stage (or should we parameterize by passing important steps to show?)

For making slides with a hacked jupman 3.5.7 I used this combo:

#jupman-purge-input
#jupman-purge-text-output

#jupman-purge-text-output is a custom command, proper implementation is tracked in #161

Probably needed:

• Preprocessor: support jupman-purge-text-output  #161