Remove custom appengine and js builds

This commit allows for gjson and sjson to be supported in
WebAssembly `wasm` projects, but breaks compatibility for
anyone using gjson with older Google App Engine and GopherJS.

Added new modifiers

`@flatten` Flattens an array with child arrays.
  [1,[2],[3,4],[5,[6,7]]] -> [1,2,3,4,5,[6,7]]
The {"deep":true} arg can be provide for deep flattening.
  [1,[2],[3,4],[5,[6,7]]] -> [1,2,3,4,5,6,7]
The original json is returned when the json is not an array.

`@join` Joins multiple objects into a single object.
  [{"first":"Tom"},{"last":"Smith"}] -> {"first","Tom","last":"Smith"}
The arg can be "true" to specify that duplicate keys should be preserved.
  [{"first":"Tom","age":37},{"age":41}] -> {"first","Tom","age":37,"age":41}
Without preserved keys:
  [{"first":"Tom","age":37},{"age":41}] -> {"first","Tom","age":41}
The original json is returned when the json is not an object.

`@valid` Ensures that the json is valid before moving on. An
empty string is returned when the json is not valid, otherwise
it returns the original json.

Removed unmarshalling

Removed unneeded comment

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