Fixed modifier pipe issue

This commit fixes an issue where chaining modifiers that used a
string arg would fail to process the modifier following the first.

fixes tidwall#143

Fix trailing multiselector value

Add link to multipaths

Fix panic when key starts at-sign

Support subqueries

It's now possible to do a query like

  topology.instances.#(service_roles.#(=="one"))#.service_version

On a JSON document such as

  {
    "topology": {
      "instances": [{
        "service_version": "1.2.3",
        "service_roles": ["one", "two"]
      },{
        "service_version": "1.2.4",
        "service_roles": ["three", "four"]
      },{
        "service_version": "1.2.2",
        "service_roles": ["one"]
      }]
    }
  }

Resulting in

  ["1.2.3","1.2.2"]

Fix missing raw result for array counts

Update README.md

Fixed chained array result

Added modifiers and path chaining

A modifier is a path component that performs custom processing on
the json.

Multiple paths can be "chained" together using the pipe character.
This is useful for getting results from a modified query.

See the README file for more information.

Valid json optimization

Added ~20% performance boost be removing extra allocation when
Valid() is called with a json string.