Expose edge-pattern on a per-edge basis

Strong (identifying) relationships are often denoted with a solid line as opposed to a dotted line. Allowing users to add this to relationships like the following would be useful:

```
Student 1--* Rating {label:"posted", style:"dotted"}
```

Start by tracing how relationship attributes are parsed and how edge styles are rendered; the issue provides the example syntax `style:"dotted"` on a relationship. Done means that per-edge style attributes are accepted and produce the requested solid or dotted relationship line in the generated diagram.

Written by the indexing model from the issue text.