Add BaseIndentWidth parser option

[Elara6331]

I'm writing a templating engine and I'm trying to add a markdown tag to it using goldmark. The syntax looks something like this:

<div>
	#markdown:
		# Heading
		[Link](https://example.com)
	#!markdown
</div>

In this case, goldmark would be given the following data as input:

		# Heading
		[Link](https://example.com)

The problem is that since there's indentation in it, goldmark parses everything as a code block. If possible, I'd like there to be a parser option to provide the base indentation width, so that the parser can ignore that and only detect code blocks if the indentation level is above the provided base level.