Purdy

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* bug fix: TwinBoxes weren't initializing proplery causing crashes when used
with AppendTypewriter

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* add Code.subset() method that returns a new Code object including only a
subset of the original's source
* upgrade minimum dependency on Pygments due to security advisory

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* Added johanvergeer's contribution of the ActionsBuilder class, a short cut
class based on a builder pattern so you can use methods on the builder class
to create actions instead of a list of actions. Supports type hinting in case
that's your thing
* Upgraded minimum version of Pygments to 2.8 due to behaviour changes in the
parser causing conflicts during testing
* Added help screen
* Added section markers and ability to skip to a section in the TUI
* Changed Sleep to accept a tuple specifying range of random amount to sleep
* Add ability to skip multiple steps, sections, or backwards steps by typing a
number before the command

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* Change base colours for URWID palette after complaints that the dark text
was hard to read against black backgrounds

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* Add ability for Transition to accept no code, so can do a screen wipe to
blank
* Add unit tests for Code source change methods
* Fix bugs found with above unit test

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* Changed how the wrapper to the pygments lexers work, the wrapper is now
responsible for choosing the palette to go with the colourizer, this means
custom colourizers for things that aren't code like (HTML, XML) can now have
different palettes from code
* Custom lexer now supports named palettes
* Add better error handling to the load action sequence
* Add error handling detecting Transitions that are missing both code and
code_box_to_copy
* Add error handling when trying to Replace outside of box boundaries
* Removed ReplaceTypewriter, multi-line replacement was ambiguous, use a
Remove and InsertTypewriter to explicitly do what you need
* Add error handling if a negative index is passed to InsertTypewriter,
updated incorrect documentation