Cleanup pager processes after closing a msgviewer

A pager is spawned every time an email is viewed but not killed off when
quitting the msgviewer, thus leading to process leakage. This patch
fixes this by adding a Close method to the msgview widget, which is
called in the close command.

Signed-off-by: Kevin Kuehler <keur@xcf.berkeley.edu>

commands/msgview/close.go

@@ -25,6 +25,7 @@ func (Close) Execute(aerc *widgets.Aerc, args []string) error {
 		return errors.New("Usage: close")
 	}
 	mv, _ := aerc.SelectedTab().(*widgets.MessageViewer)
+	mv.Close()
 	aerc.RemoveTab(mv)
 	return nil
 }