Add metadata to syntax highlighter

[lucperkins]

I'd like to use Comrak to create some custom Markdown extensions, including fancy code blocks along the lines of rehype-pretty-code. One barrier presented by the current implementation is that SyntaxHighlighterAdapter doesn't provide access to arbitrary metadata like this:

Here is some Rust code:

```rust filename="main.rs"
fn main() {}

Here, filename="main.rs" wouldn't be accessible to SyntaxHighlighterAdapter. Now, it is possible to access this in Comrak using the more manual method (iterating through the AST nodes) but I find SyntaxHighlighterAdapter much more ergonomic. Happy to help out with this, just wanted to see if others had run into this limitation.

[kivikakk]

Per #257, #253 #276 hopefully now covers this!