Add html id to headers so that you can link to specific headers in the markdown.

[markcho]

Currently, generated html doesn't include any anchors or ids on the header.

This PR uses the Github-style slugs for header ids so that links to headers now work. This is useful for things like table of contents at the top.

[jfernandez]

Good feature. Main ask: drop the regex dependency. Since markdown-rs produces clean HTML (no attributes on headings), this can be done with str::find for the heading tags and a simple char iterator to strip inner HTML. That avoids pulling in aho-corasick, regex-automata, and regex-syntax for two simple patterns.