support more file formats

[denizyuret]

See JuliaIO/JSON.jl#276 for JSON and JuliaIO/BSON.jl#24 for BSON. If we can get them to play with FileIO, Knet can use them. Also take a look at MAT, HDF5, NPZ etc. ONNX support is more complicated, separate issue #302.

[denizyuret]

Update 20190502:

• ONNX: See https://github.com/KnetML/ONNXKnet.jl for ONNX support.
• JSON: FileIO compatibility JuliaIO/JSON.jl#276: does not seem like JSON is going to support FileIO, though JSONFileIO may be a good idea.
• BSON: support was added in BSON and FileIO but they chose to adopt a different "save" convention ("a"=>a,"b"=>b) instead of ("a",a,"b",b), which right now prevents out of the box support for BSON in Knet.