Handle Hugo style anchors in heading text

[vedang]

The static site generator Hugo allows the following ways of defining heading anchors in Markdown:^1

• ## Heading text {#heading-text-anchor}
• ## Heading text {id=heading-text-anchor}

When the Markdown already has these anchors built in, they are not handled by markdown-clj currently.

This commit adds handling for this style of custom anchor definition when making headings.

Testing:

• Manual export of markdown content with custom anchors is working
• Manual export of markdown content without custom anchors is working (anchor is generated)
• clojure -M:test all tests passed

[yogthos]

Thanks, just pushed out 1.11.2 with both PRs to Clojars.