Support passing command line options to phantomjs

My use case was I wanted to enable the disk cache. So my webshot option look like:

```javascript
{phantomConfig: {"disk-cache": true}}
```

I wanted to pass a path to a JSON file (as phantom says it supports this via the --config param) but phantom did not read or set configuration options when I did this.

In the future when phantomjs has fixed the bug, webshot can support this by passing a string as the phantomConfig option but until then this'll have to do.