fixes based on pull-request comments.

- document text.
- Merged Incrs() feature within Incr().
- Added a similar feature for Decr().

- Incr() and Decr() operates on map[string]inteface{}, and value
  can be either []interface{} or []float64{}. There is no way to
  handle them like generics.

refer:
    github.com/dustin/go-jsonpointer/pull/5