Add support for directed hypergraphs

[espottesmith]

The main point of this PR was to add support for directed hypergraphs to SimpleHypergraphs.jl, directly addressing #45. To do this, I ended up making some abstract types, including AbstractHypergraph, AbstractUndirectedHypergraph, and AbstractDirectedHypergraph, which makes this package more flexible, and I also added the concrete types BasicHypergraph and BasicDirectedHypergraph which don't contain metadata.

I've added tests (I think with good coverage) and updated the docs. I have not created an example or tutorial yet.