Custom data-readers

[chrisrink10]

Clojure supports custom data readers using a search for data_readers.cljc in the Java classpath. We can do that with PYTHONPATH/sys.path instead.

It may also be nice to support a more structured format using importlib.entry_points allowing Basilisp libraries and modules to offer customized data readers without requiring the extra effort of a custom data_readers.cljc. For entrypoints data readers, we should allow a CLI flag to disallow using entrypoints readers.