Add a network diagram (nwdiag) exporter based on the deployment specification

[dgutson]

This issue supersedes structurizr/dsl#183
@qequ is developing this new export format, which will create a BlockDiagram's nwdiag diagram based on the information of the deployment view and the associated deploymentEnvironment.

@simonbrowndotje if you like it, you can add it to structurizr once finished. The result so far is very promising and provides more concise and qualitative information than the regular deployment diagram.

Mapping specification:

• one network diagram per deployment view/environment
• one network group per deployment node
• one network element per container instance
• networks are specified as tags of the container instance
• elements belonging to the same software system have the same color. This is because there are no nested groups in the nwdiag to denote the two levels.
• (next version) shape is obtained from the element styles. BlockDiag doesn't have the cylinder shape but we will add it.

The reason we chose the original BlockDiag's nwdiag over the PlantUML's one is because of limitations of the latter, such as not being able to have colored nodes, and lacking of validations. It's easier to add the cylinder shape to BlockDiag than to fix the coloring of the node background.