Add a command line option to jsontoh5 to specify the library version when creating a file

Currently every HDF5 file produced with `jsontoh5` will use the latest file format, `H5F_LIBVER_LATEST`: https://github.com/HDFGroup/hdf5-json/blob/develop/h5json/hdf5db.py#L167. This creates a problem for software that still does not use, or never will, the latest version of the HDF5 library.

`jsontoh5` should have something like `--libver` option with either `earliest` or `latest` for values. I don't think that h5py yet supports all the new options described here: https://portal.hdfgroup.org/display/HDF5/H5P_SET_LIBVER_BOUNDS.