Add support for markdown="0|span|block" attribute in HTML blocks

[osteele]

Description

Currently, gojekyll does not support the markdown="1" markdown="0", markdown="span", or markdown="block"` attribute in HTML blocks, which are supported by Jekyll's default Kramdown processor.

Background

This issue was originally reported as part of #51, where it was mentioned that while the README documents that markdown="span" and markdown="block" are not supported, it does not explicitly mention that markdown="1" is also not supported. markdown="1" are currently supported, but markdown="0", markdown="span", and markdown="block" are not.

Expected Behavior

Add support for markdown="0", markdown="span", and markdown="block" attributes in HTML blocks, similar to how Jekyll's Kramdown processor handles them.

Example

<div markdown="0">
This is *markdown-like* content inside a non-HTML block.
</div>

[osteele]

According to the Kramdown syntax documentation, the markdown attribute values have the following meanings:

• 0: Disables markdown processing entirely
• 1: Enables markdown processing (default)
• block: Parses the content as a block-level element
• span: Parses the content as a span-level element

These values control how the content within the element is processed by the markdown parser.

[osteele]

The code and tests already support markdown="1". Kramdown also supports markdown="0", while this gojekyll doesn't. I've expanded the title to address this.