PipeCommand: pass shell arg to ExternalCommand.

Prior to this branch, `shell=True` was passed to Popen regardless of
whether `--shell` was passed. I'm not sure why this was the case but it
wasn't a problem. Now that we're passing to ExternalCommand, this
doesn't work because ExternalCommand does it's own manipulation of cmd
when shell=False which is incompatible with a list-style cmd.

alot/commands/thread.py

@@ -801,7 +801,7 @@ def callback(out):
 
             await ui.apply_command(ExternalCommand(cmd,
                                                    stdin=mail,
-                                                   shell=True,
+                                                   shell=self.shell,
                                                    thread=self.background,
                                                    on_success=callback))