Only process code fences with a 'process' flag present

[brendanzab]

Hey, thanks for the preprocessor!

I've noticed that mdbook-graphviz requires a process flag to be present before processing them as graphviz diagrams:

```dot process
digraph {
    processed -> graph
}
```

Just as a matter of 'preprocessor hygiene', it would be really cool if mdbook-svgbob did similar! This would mean you could talk about bob documents as well as processing them as SVGs. I realise this is a breaking change though.

[boozook]

Hey 👋
That's easy change but I have a questions. If it steel relevant off course.

I'm not quite sure what it's for exactly. Could you explain purposes of this?