Relocating config.yaml for bridges

At the moment, at least on my macOS machine, it looks like the `config.yaml` for the bridges gets installed somewhere like this:

```
/Users/ec/Library/Application Support/bbctl/prod/sh-imessage/config.yaml
```

It'd be nice if this were configurable - there's a `--config-file` argument, but it seems to only allow subdirectories of that default directory.