support for cpp

hello! I got this error while running cdxgen

```
Failed to run atom-tools. Use the atom container image or perform 'pip install atom-tools' and re-run this command. Exception:
Unknown origin type: cpp
```

so I tried to run it manually to confirm the behavior
```shellsession
$ atom-tools convert -t cpp

Unknown origin type: cpp
```

looks like cpp isn't supported, right?

https://github.com/AppThreat/atom-tools/blob/d1461e61a1bc82ce39c735e0102f953e6d647e8f/atom_tools/cli/commands/convert.py#L88