The user can configure a list of addresses that are associated with a
context by passing a list to set-email-account!, e.g.
(set-email-account! "foo"
...
(+mu4e-personal-addresses ("foo@bar.org" "bar@baz.org")))
This list, if it exists, is used to prompt the user for the "from"
address after trying to set the "from" address automatically.