CasperJS options can be passed in via grunt task. 

CasperJS options can be passed in via grunt task. There are no restrictions now as to which options can be used.

This allows for more flexibility and less less maintenance by allowing user defined options and not restricting the options to hard coded set.
  
I've updated the README as well. 

**Note**: This breaks how options were previously passed in.

Resolves the issue raised in ronaldlokers/grunt-casperjs#33