added exporting.csv.url option

no more privacy concerns and 

https://raw.github.com/highslide-software/highcharts.com/master/studies/csv-export/csv.php

says

```
/**
 * DISCLAIMER: Don't use www.highcharts.com/studies/csv-export/csv.php in 
 * production! This file may be removed at any time.
 */
```