Set output destination for generated files

By default, to get a file generated by ScanCode, we do need explicitly set filename and/or directory, like:

```
--spdx-tv myfile.tv
--json-pp somedir/myfile.json
```

This presents a problem as when used as underlying tool, the command line config can't be changed all the time

Instead, a common output destination should considered by default as a command line, as together default naming for files, then reducing the overload to type naming for all files. Ex.:

```
--output-content somedir/somesubdir --basename myproject --spdx-tv --json-pp
```
This would end up generating **somedir/somesubdir/myproject.spdx.tv** and **somedir/somesubdir/myproject-prettier.json**

- [x] Enhancement
- [x] Scan Feature