Use <yargs> to parse command line args, and support JSON config

Fix #8 

```bash
thumbsup --config config.json
```

**config.json**

```js
{
  "input": "/media/output",
  "output": "./website",
  "title": "My holiday",
  "thumb-size": 200,
  "large-size": 1500,
  "css": "./custom.css"
}
```