Autoupdate by configuration.

This makes possible to autoupdate gh when `autoupdate` is set to `true` in the configuration.

It changes the configuration file to look like this:

```json
{
  "autoupdate": false,
  "credentials": [
    {"username", "token", "host"}
  ]
}
```

But it keeps compatibility with the old configuration format. So it should still load your credentials if your configuration looks like this:

```json
[
  {"username", "token", "host"}
]
```