mu4e: Add possibility to forward messages as attachments

This commit adds a global variable
mu4e-compose-forward-as-attachment. To enable choosing forwarding
method on a per-message basis would probably require either:

• changing the mu server backend so that it distinguishes between
  inline forwarding and forwarding as attachment;
• changing the mu server backend so that it doesn’t return attachments
  at all and making both inline and as attachment forwarding via
  MIME (and also making mu4e actually display MIME-enclosed inline
  emails).

mu4e/TODO

@@ -11,7 +11,6 @@
 *** message statistics
 *** include exchange handling / org integration
 *** integrate bbdb
-*** forward-as-attachment
 *** identity support