Allow more leading # in ATX heading as a customization option

[yufeih]

We are using TOC.md to represent table of contents, It is a restricted markdown format that only allows several known markdown syntax, like links and ATX headers.

Sometimes these files can be very big and exceed maximum leading count as defined by common mark spec, like this one,

The ask is if it is possible to have an option in HeadingBlockParser that can override the default max leading count (6), so we can reuse HeadingBlockParser to parse our special TOC markdown.

Related dotnet/docfx#3874

[xoofx]

Sure, via an extension and a slight change on HeadingBlockParser, PR welcome

[OsmondJiang]

Thanks a lot for quickly merging the PR 👍, it would be great if we can know when we can get a new nuget version?

[xoofx]

It should be come with upcoming 0.15.7 in a few minutes