Purdy

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* Added HighlightChain action
* Added VirtualCodeBox and ability to copy a VCB into a real code box through
a Transition action
* Used the iscreen mechanism introduced in the last release to create a better
test harness

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* Bug fix: purdy cmd wasn't launching properly due to type-o in fake args

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* Refactored how screens work, they're now a proxy for an implementation
inside of "purdy.iscreen". The Urwid code viewer now lives in
"purdy.iscreen.tui" and a new viewer has been added that does text export
* Added Sleep action
* Tranistion actions are now skippable
* Transition actions don't automatically trigger a Wait anymore, you have to
call Wait explicitly

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* Bug fix: crash when Fold is called without a Wait immediately afterwards,
urwid was caching a focus position and trying to set it to a line that
wasn't there anymore

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* Bug fix: crash when fast-forward called on Transtion due to missing skip
parameter

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* Breaking change release
* Moved animation mechansim to be based on a queue, can now go forwards and
backwards
* Signatures of Actions and Screens have changed
* Add tools for printing code in other formats such as RTF and HTML