pass toJSON options and add option to included computed fields

Addresses #17 by sliceing off any extra arguments and passes them to the original `toJSON` function.  

Also inspects those options to see if `computedFields` was set.  If so, short-circuits the `_.reduce` so the `toJSON: false` option will be ignored.

If this looks good, then I can update the README and even add a test case before you merge it.  The tests pass now, but there's no case for the new option.