'title' parameter of 'render' block should be caption of included diagram

i.e., the following:

  ```{render="{{plantuml}}" title="Alice & Bob"}
  @startuml
  Alice -> Bob: hello
  @enduml
  ```

...should result in this:

```
![Alice & Bob](.build/cache/0ec62f1568ac33e20ec8d430ae77a9cbe6c9cd46.svg "Alice & Bob")
```
...rather than this:

```
![](.build/cache/0ec62f1568ac33e20ec8d430ae77a9cbe6c9cd46.svg "Alice & Bob")
```