topologicalSort produces invalid results for graphs with cycles

Performing a topological sort on a graph with cycles should throw an error (related: https://github.com/datavis-tech/graph-data-structure/issues/32).

Currently, you can do:

```javascript
g = Graph()
g.addEdge('a', 'b').addEdge('b', 'a').topologicalSort()
// => [ 'a', 'b' ]
```