Support extra code tags

[jonathanpallant]

The mdbook tool supports code blocks like:

```rust

But also ones like

```rust,editable

You typically use ,editable to get line numbers on the code block (but it also makes the code editable in-line).

The latter kind are not supported by mdslides currently - the language goes though as rust,editable which is not supported by highlight.js.

We could try and patch mdbook to make all code blocks editable by default, or we should patch mdslides to strip ,editable from any code blocks.

[jonathanpallant]

Ah, so this works:

```rust editable
fn your_code_here() { }
```

[jonathanpallant]

Here's a concrete example:

```rust compile_fail
use std::io::prelude::*;

fn main() -> std::io::Result<()> {
    let f = std::fs::File::create("hello.txt")?;
    write_and_close(f);
    write_and_close(f);
    Ok(())
}

fn write_and_close(mut f: std::fs::File) {
    f.write(b"hello");
}
```