Adding optional parameters to ouibounce.disable()

Added 2 optional parameters to the .disable() function which allow:

- To change the scope of the cookie between site wide or only the current page. 
- To modify the cookie expiration length. (This would allow a user to use the disable function in different contexts. ie successful form submision and modal exit would have different expiration lengths etc.

Added in a demo for the less aggressive behavior and modified readme.md to include the two parameters along with example code.