Add read_json and list_json_columns for JSON support. (#412)

Author: jiachengxu

tensorflow_io/json/BUILD

@@ -10,7 +10,7 @@ load(
 cc_library(
     name = "json_ops",
     srcs = [
-        "kernels/json_input.cc",
+        "kernels/json_kernels.cc",
         "ops/json_ops.cc",
     ],
     copts = tf_io_copts(),

tensorflow_io/json/__init__.py

@@ -15,6 +15,8 @@
 """JSONDataset.
 
 @@JSONDataset
+@@list_json_columns
+@@read_json
 """
 
 from __future__ import absolute_import
@@ -24,9 +26,13 @@
 from tensorflow.python.util.all_util import remove_undocumented
 
 from tensorflow_io.json.python.ops.json_ops import JSONDataset
+from tensorflow_io.json.python.ops.json_ops import list_json_columns
+from tensorflow_io.json.python.ops.json_ops import read_json
 
 _allowed_symbols = [
     "JSONDataset",
+    "list_json_columns",
+    "read_json",
 ]
 
 remove_undocumented(__name__, allowed_exception_list=_allowed_symbols)