disable webp support by default and add config option

Fixes #109 

This config file can be used to turn webp back on:

```json
{
    "generator-assets":  { 
        "svg-enabled": false,
        "webp-enabled": true
    }
}
```

Name as `generator.json` in CWD.

Also documented this config option in https://github.com/adobe-photoshop/generator-assets/wiki/Configuration-Options