FR: configuration to import custom python functions

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](https://docs.python.org/3/library/sqlite3.html#sqlite3.Connection.create_function).

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`](https://stackoverflow.com/q/67631), 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!