Support more JSONPath features

[dumbmatter]

There are many features in the JSONPath spec that we currently don't support, such as:

• array slices (stackToPathArray will need to get more specific than assuming every array is "wildcard")
  • index
  • negative index
  • slice with one side unbounded
  • slice with negative index and one side unbounded
  • slice between two numbers
  • step size like 1:2:9, in all of the above situations
• filter expressions (starting with ?)
• functions at tail end of path (like min, max, etc)
  • can there be multiple?
  • @ referencing this object
  • $ referencing root object
• .. deep scan

I think most people (myself included) won't actually use those features, but maybe someone would. And they'd be kind of cool, and some of them not that difficult to implement.