Make default config options easier to discover

That's an UX suggestion.

At the moment, the default config file has `"PublicGateways": null`, which is implicitly replaced by:
```
"PublicGateways": {
"localhost": {
"Paths": ["/ipfs", "/ipns"],
"UseSubdomains": true
}
}
```

If you don't know this, you can easily think that the behavior of go-ipfs would be the same for any request, right ? But if you try to make requests with anything else than `localhost`, the gateway will not redirect to the CID subdomain.

I understand that it makes technical sense but it's easily confusing. Having the default value set in the config file when initialized would help.

On a side note, it would be useful to support wildcard to define those subdomains instead of listing every possible variant.