Implemented Components function

[orzel7]

Resolves #163

[bodgit]

This doesn't appear to be working fully, unless I'm misunderstanding something. Consider this example:

g := graph.New(graph.StringHash)

_ = g.AddVertex("A")
_ = g.AddVertex("B")
_ = g.AddVertex("C")
_ = g.AddVertex("D")
_ = g.AddVertex("E")
_ = g.AddVertex("F")
_ = g.AddVertex("G")
_ = g.AddVertex("H")

_ = g.AddEdge("A", "B")
_ = g.AddEdge("A", "C")
_ = g.AddEdge("A", "D")
_ = g.AddEdge("B", "C")
_ = g.AddEdge("C", "D")
_ = g.AddEdge("D", "E")
_ = g.AddEdge("E", "F")
_ = g.AddEdge("E", "G")
_ = g.AddEdge("E", "H")
_ = g.AddEdge("F", "G")
_ = g.AddEdge("G", "H")

file, _ := os.Create("graph.dot")
_ = draw.DOT(g, file)

_ = g.RemoveEdge("D", "E")

components, _ := Components(g)

for i, component := range components {
	file, _ := os.Create(fmt.Sprintf("component%d.dot", i))
	_ = draw.DOT(component, file)
}

This creates a graph like so:

I then remove edge D-E which should split this into two separate graphs while still retaining every other edge, but I end up with these:

Many of the edges are now missing; A-C, A-D & C-D from one graph, and F-G & G-H from the other. It varies slightly depending on which vertex is used as a starting point but it always loses some edges.