Consider normalizing To/From/Cc etc contents

[simonw]

The To: and Cc: headers are currently stored as text:

https://github.com/simonw/mbox-to-sqlite/blob/9c1e65a1c353572c4b0262236537559c548723cd/tests/test_mbox_to_sqlite.py#L35-L37

Maybe normalize these and extract them out nto many-to-many tables?

This would also need to handle Name <[email protected]> formatted data.

[simonw]

Also an opportunity to clean up that \n\t in [email protected], \n\[email protected]".