Add a multi-email mode

[CydeWeys]

It should hopefully be reasonably simple to change the email config option from being a singleton to being a list, thus allowing multiple email accounts to be read from and combined into a single order tracking spreadsheet. The rest of the tool would hopefully be the same, the only difference being, in the "get shipping emails" step, running that in a while loop and connecting to multiple accounts.

This is not too dissimilar from my early implementation of multiple matching email keys per buying group.

[CydeWeys]

And I'm just realizing that the issue title isn't great, and it's not so much a new mode as enabling to be configured multiple email addresses.