Add HDF5 loader

[adamslc]

Some complications that this loader might introduce:

• a single .h5 file frequently corresponds to several Datasets, and it would be nice if the specification in Data.toml could refer directly to a single DataSet, but preserve the hierarchal relationship between multiple DataSets.
  • More generally, I frequently work with a directory full of .h5 files, and it would be nice to preserve the relationship between all of these files as well.
• In HDF5, a dataset can have attributes, and it is important that these be accessible from DataToolkit
• The HDF5.jl package is relatively heavyweight, and so it would be good to not take on an explicit dependency of this package.