Forward email with shortcut 'forward' (default ctrl+f)
This opens a composing tab letting you to select receiver etc.
"composing" config setting "forward_as_attachment" selects the
forwarding behavior:
- "ask" asks you ever time
- true always forwards by attaching the entire email as a single
attachment
- false always forwards by inlining the email, like most email clients
do.
Closes#120