as per the discussion https://lists.sr.ht/~sircmpwn/aerc/patches/15367
this handles completions in `completer/completer.go` by enabling the
completer to return a `prefix` that will be prepended to the selected
completion candidate.
When the list of completions from the external command doesn't have
associated contact names, the email address we attempt to parse was
being terminated with a newline. Now, we strip the trailing newline if
present.
When addresses contain special characters, net/mail MIME-encodes them
to a valid RFC 5322 address for use in headers. However, these are not
pleasant for human consumption, so we decode them for use in the
completion list. Aerc properly encodes addresses when the message is
sent.
This patch also removes surrounding white space from contact names, if
present.
