When updating voltron don't overwrite the current config file

My proposed way of doing this is as follows:

- Provide a `voltron.default.cfg` file that is always overwritten on install.
- After installation, if `voltron.cfg` doesn't exist, then either fallback to `voltron.default.cfg` at runtime or copy `voltron.default.cfg` to `voltron.cfg` immediatly after installation.

I have currently only implemented the first part, as I don't know how to implement the second part with setuptools and I think you should decide, which of the two possibilities is the most desirable.