Add support for edge attributes about graph

[dominikbraun]

Change: Instead of supporting a single label, a map of attributes should be supported. This is a map[string]string and allows for better flexibility and extensibility.

The functional option should be EdgeAttribute(key, value string), resulting in a call as follows:

g.Edge("A", "B", graph.Weight(5), graph.EdgeAttribute("color", "red"))

These attributes will be rendered by #20.

[dominikbraun]

#20 already introduces the Attributes field.