New Option - customCSS

This PR adds a new `customCSS` option that allows a user to pass custom CSS rules that will be applied to a webpage before the screenshot will be taken.

**Example**

```javascript
webshot('http://google.com', 'screenshot.png', {
  'customCSS': 'body { background: blue !important; }'
}, function(err) {

});
```

**Note:**  I wasn't sure how to adequately test this with Mocha and ImageMagick, so I only verified that it works manually.