tfr-read: transparently support stats files (currently just schema files)

[brianmartin]

From @daikeshi https://github.com/spotify/spotify-tensorflow/pull/127#issuecomment-452423844:

[tfr-read] assumes that there'll be _schema.pb or _inferred_schema.pb file stored with the dataset, but I think we are now moving to use stats file or schema txt, we should probably change tfr-read to reflect that.

tfr-read should output json data:

• using schema txt file if it's provided
• using stats file if there's a stats file with the data, but no explicit schema txt file argument
• using the default ExampleDecoder if no schema file and stats file are provided