Allow to set the parameter file format

[TomonobuTsujikawa]

The parameters in the nnp file are stored in h5 format by default. This PR tends to add an option to allow include a .protobuf file in nnp file.

Here is the example:

# These are stored by h5 format:
save('model.nnp', contents=contents)
save('model.nnp', contents=contents, parameter_format='.h5')

# This is stored by protobuf format:
save('model.nnp', contents=contents, parameter_format='.protobuf')