Allow custom DataTables by passing functions for .next and .hasNext

[mitalia]

As it stands, DataTables have fixed .next and .hasNext functions. Ideally, an advanced user (or someone doing work on the internals of DataExpress) might benefit from a constructor that allows one to pass a custom function to .next and .hasNext. Otherwise, the only other way to do this is to subclass DataTables and override these methods.