Option to set exit status code when query doesn't yield any results

**Problem Statement**

There's currently no programmatic way of testing if a node has been found.

It would be nice to add an option (maybe `-s`) so xq return an exit status code depending on wether a node has been found or not, à la grep.

**Expected Result**

```sh
> echo '<?xml version="1.0"?><root></root>' | xq -s -q none
> echo $?
1
```