Support an `X-ExtEditorR` meta header

[Frederick888]

Description

There are now 7 (and growing) X-ExtEditorR- headers. They take quite some space.

From: John Smith <foo@example.com>
To: 
Cc: 
Bcc: 
Reply-To: 
Subject: 
X-ExtEditorR-Priority: normal
X-ExtEditorR-Delivery-Format: [auto]
X-ExtEditorR-Attach-vCard: [false]
X-ExtEditorR-Delivery-Status-Notification: false
X-ExtEditorR-Return-Receipt: false
X-ExtEditorR-Send-On-Exit: false
X-ExtEditorR-Allow-X-Headers: false

We can have a meta X-ExtEditorR header to remedy:

From: John Smith <foo@example.com>
To: 
Cc: 
Bcc: 
Reply-To: 
Subject: 
X-ExtEditorR: Priority:        normal, Delivery-Format:             [auto]
X-ExtEditorR: Return-Receipt:  false , Delivery-Status-Notification: false
X-ExtEditorR: Attach-vCard:   [false], Send-On-Exit:                 false
X-ExtEditorR: Allow-X-Headers: false

The alignment would be quite annoying to implement, and it's gonna be messy anyway when a user edits them. So still thinking if this is actually worthwhile.