output not splitted to stdout and stderr when both --json and --verbose are used

wouldnt it make sense to split the output to stdout and stderr when using both `--json` and `--verbose` options so that we could do:

```
./librespeed-cli --json --verbose > result.json 2> librespeed.log
```

right now everything is mangled into stdout, if a person wants that behaviour with (both option set), they would then do

```
./librespeed-cli --json --verbose &> result.txt
```