sed should be silent if '-i' is provided

```sh
$ sed -i 's/foo/bar/g' file.txt # note: this is silent
$ cat file.txt
bar
$ shx sed -i 's/bar/baz/g' file.txt # this should be silent
baz
$ shx cat file.txt
baz
```

We could arguably make the change in shelljs itself (which would help `n_shell`).