Allow any number of spaces before codeblock

[jez]

This allows using code blocks in lists, like

- Some list item:

  ```ruby
  class A; end
  ```

- Another list item:

  ```ruby
  def foo; end
  ```

I can't quite tell the original motivation for requiring four spaces, but I have
been using this change locally for multiple weeks and it seems to work fine for
me.

Before

After

[jez]

👋 @fmoralesc sorry to bother you; do you maybe have a moment to look at this PR and also #380 ?

Thanks in advance 🙏

[fmoralesc]

LGTM. I can't remember why there was a need for the 4 spaces either.

[bewuethr]

This has been bothering me forever 😅 thank you! 🚀