Consider normalizing To/From/Cc etc contents

[simonw]

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

mbox-to-sqlite/tests/test_mbox_to_sqlite.py

Lines 35 to 37 in 9c1e65a

	"To": "tim.belden@enron.com, kevin.presto@enron.com, john.lavorato@enron.com",
	"Subject": "12/28 Power VaR",
	"Cc": "frank.hayden@enron.com, stacey.white@enron.com, lacrecia.davenport@enron.com, \n\tdavid.port@enron.com",

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

This would also need to handle Name <email@example.com> formatted data.

[simonw]

Also an opportunity to clean up that \n\t in lacrecia.davenport@enron.com, \n\tdavid.port@enron.com".