Option to use triple-backticks for codeblocks

[casey]

I prefer triple-backticks for codeblocks, since i find it a little easier to edit. What would you think of an option to use triple-backticks for codeblocks, instead of indentation?

[kivikakk]

Do you mean like this?

Here's my code block:

```go
fn main() {
    println("hi")
}
```

It's already supported, in base CommonMark; see https://spec.commonmark.org/0.29/#fenced-code-blocks.

[casey]

Ah, sorry, I should have been more precise, I was referring to an option to use backticks for codeblocks when dumping a parsed tree as CM, since parsing fenced codeblocks is already supported.

[kivikakk]

Aaaaah, yes. I think that's an excellent idea! You'll note the renderer already does this if there's an "info" string attached to the code block (for fenced code blocks like ```ruby), so you could plug in the option at the same point to use the fenced style.

The ComrakOptions struct is getting crowded. Feel free to add it in, but also feel free to maybe decompose it a bit? (e.g. parse vs output options)

[kivikakk]

See #139 re: refactoring the options struct.