[lzjson] added lzjson_path_get (like xpath)

if you have a json that looks something like:

{
	"a" : "b",
	"c" : [ 1, 2, 3, { "foo" : 1 } ]
}

You can get specific values from this via lzjson_path_get, for example:

lzjson_path_get(json, "a") would return "b",
lzjson_path_get(json, "c.[3].foo") would return '1'

Array indexes are defined via brackets, while a object key is just a string.
Each sub-object is a "."