feat: allow auto rendering code snippets

[mfontanini]

This adds a new auto_render_languages option (meaning it can be configured in the config file or in the presentation itself) that specifies a list of languages for which the +render attribute is implied and are therefore automatically rendered. This means for example this presentation would have its mermaid diagram automatically rendered as an image:

---
options:
    auto_render_languages:
        - mermaid
---

```mermaid
sequenceDiagram
    bob ->> mike: oh hi
```

Fixes #416