Edge attributes support

[ki11roy]

First, I would like to say thank you for such a handy tool, because I personally hate to draw diagrams by hand.
This PR adds support for Edges, colors and labels in particular and some other features like circular references and so on. Also, I have slightly changed tests to use folders instead of files in some cases. Just comment tear down to keep the folders with images.

Examples of usage could be seen in the tests, for example:

                node1 = Node("node1")
                node2 = Node("node2")
                nodes = [Node("node3"), Node("node4")]
                
                # line                
                nodes - Edge(color='red') - node1

                # one directional
                nodes >> Edge(color='red', label='6.1') >> node1
                nodes << Edge(color='green', label='6.2') << node1
                
                # both directional
                node1 << Edge(color='blue', label='1.3') >> node2

                # loop
                node >> Edge(color='blue', label='3.3') << node

[ki11roy]

As an improvement Edge could be replaced with dict in some cases, just to add some consistency with the original syntax ( node1 >> {'color': 'red'} >> node2 ), but it won't work with the group of nodes, regretfully ( nodes >> {'color': 'red'} >> node ).

[pchaganti]

@ki11roy Is it possible to assign an icon to a node?

thx!

[ki11roy]

@pchaganti take a look at examples with Custom Nodes, this feature is about edges only.

[mingrammer]

Awesome.

Great works! I'll test it with some diagrams for checking backward compatibility.

And could you please add a practical example diagram in the examples page?

[ki11roy]

Added some examples and additional parameter "style" (i.e. dotted, dashed, bold, solid).

[mingrammer]

LGTM!

Thank you for adding a great feature. I really appreciate it :)