HTMLFormatter ignores links in title

[relikd]

This is currently not detected as link (but should be):

## [0.0.1] - 2014-05-31

[0.0.1]: https://github.com/olivierlacan/keep-a-changelog/

EDIT: when called in HTMLFormatter.format() ... the Document seems to correctly identify the link

[relikd]

ok, I found the reason:

swift-markdown/Sources/Markdown/Walker/Walkers/HTMLFormatter.swift

Lines 92 to 94 in 55d66d9

	public mutating func visitHeading(_ heading: Heading) -> () {
	result += "<h\(heading.level)>\(heading.plainText)</h\(heading.level)>\n"
	}

A heading does not descend into nested Markup.

... question on the side: What is the intended way to customize the HTML Walker? Since it is a struct, I cant just subclass it and overwrite a single method. Do I really have to copy the whole implementation to modify it?