code: Show both stdout and stderr

Code blocks may try to print to stderr which is currently dropped.

internal/code/code.go

@@ -121,7 +121,7 @@ func Execute(code Block) Result {
 		}
 		// execute and write output
 		cmd := exec.Command(command[0], command[1:]...)
-		out, err := cmd.Output()
+		out, err := cmd.CombinedOutput()
 		output.Write(out)
 		if err != nil {
 			output.Write([]byte(err.Error()))