Add nimblepath so Nim can find Nimble-installed packages out-of-the-box

Currently, `bigbreak` doesn't set the correct `nimblepath` in its default config (nim.cfg). This patch adds a nimblepath to the config file, avoiding the following class of errors:

```bash
file.nim(1, 7) Error: cannot open 'module'  
```

Where `module` is any nimble-installed package.