Enables vertices to have more than one label

[MachinesAreUs]

I know it's possible to assign complex labels to vertices, like [:foo, :bar], but it would be nice if the handling of multiple labels could be internal to the library, since in several use cases you don't know beforehand all the labels for vertices.

This could work like this (modified and extended from doctests for Graph.label_vertex/3):

iex> g = Graph.new |> Graph.add_vertex(:a, :foo)
...> [:foo] = Graph.vertex_labels(g, :a)
...> g = Graph.label_vertex(g, :a, :bar)
...> Graph.vertex_labels(g, :a)
[:foo, :bar]

iex> g = Graph.new |> Graph.add_vertex(:a, [:foo, :bar])
...> Graph.vertex_labels(g, :a)
[:foo, :bar]

You could also receive both single terms and lists in Graph.label_vertex/3:

iex> g = Graph.new |> Graph.add_vertex(:a)
...> g = Graph.label_vertex(g, :a, [:foo, :bar])
...> Graph.vertex_labels(g, :a)
[:foo, :bar]

This implies 2 breaking changes:

• The s added at the end of the original Graph.vertex_label/2.
• Graph.vertex_labels/2 always returns a list.

I was careful not to break the .dot serialization (at least with the current tests), but maybe it is worth discussing including both the vertex name and labels in the generated .dot file.

[coveralls]

Coverage decreased (-0.1%) to 94.775% when pulling 0d47b62 on MachinesAreUs:feature/multiple_labels_by_vertex into 391b24d on bitwalker:master.

[coveralls]

Coverage decreased (-0.1%) to 94.775% when pulling 9b83eb7 on MachinesAreUs:feature/multiple_labels_by_vertex into 391b24d on bitwalker:master.

[coveralls]

Coverage increased (+0.06%) to 94.946% when pulling c79b5c0 on MachinesAreUs:feature/multiple_labels_by_vertex into 391b24d on bitwalker:master.

[coveralls]

Coverage increased (+0.06%) to 94.946% when pulling 74733e2 on MachinesAreUs:feature/multiple_labels_by_vertex into 391b24d on bitwalker:master.

[MachinesAreUs]

Hi @bitwalker. Any thoughts on this? :)

[bitwalker]

Sorry for the delay, I promise this is next on my list! Hopefully review this weekend and go from there :)

[bitwalker]

Can you rebase your changes on master? There are some changes in your PR that I've made on master, or that I would prefer not be in PRs (such as the version change), but other than that, it's pretty much ready to merge. Sorry for the long wait!

[coveralls]

Coverage increased (+0.06%) to 94.955% when pulling 663b853 on MachinesAreUs:feature/multiple_labels_by_vertex into 3851394 on bitwalker:master.

[MachinesAreUs]

Ready @bitwalker ! :)

[bitwalker]

Thanks! Since this is a breaking change, I'll release it as 1.0.0 sometime today.