FR: configuration to import custom python functions

[mjpieters]

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LiteCLI is built in Python, and the Python bindings for SQLite make it possible to bind custom functions you can then call from SQL, see the Connection.create_function() documentation.

I'd love to see litecli support defining your own custom functions for use in SQL queries. This could be in the form of compiling the source code for such a function embedded in the config file, or to support importing functions from Python modules (I'd suggest a path:callable syntax, where you use importlib to import path, then get the callable name from the imported module to register as a functoin).

Bonus points if you can also register aggregation, window and collation functions!