add the ability for the user to customize their day

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There are roughly two options here:

• add a "Preferences" window that can allow for the selection of start of day, end of day, length of pom, length of break, etc
• add the ability for the user to write a little Python plugin file that runs a function which constructs a Day object for a given datetime.date

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the latter is probably a bit easier to implement and more flexible, although it has obvious usability and security implications

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This is obviated by #36