Add signatures

This adds the ability for per-account signatures in the accounts.conf
config file. The signature is added to emails in the editor at the
bottom of the email. This includes when forwarding, replying to, and
composing emails.

There are two config options: signature-file and signature-cmd. The
former allows a signature to be read from a file and the latter allows
an arbitrary command to be executed to return the signature.

The config options have been documented in aerc-config

commands/msg/unsubscribe.go

@@ -88,6 +88,7 @@ func unsubscribeMailto(aerc *widgets.Aerc, u *url.URL) error {
 		"Subject": u.Query().Get("subject"),
 	}
 	composer := widgets.NewComposer(
+		aerc,
 		aerc.Config(),
 		acct.AccountConfig(),
 		acct.Worker(),