This enables the smart-punctuation setting by default. The long term
plan is to continue to enable more markdown extensions by default across
semver breaking releases.
This adds `MarkdownOptions` for creating the pulldown-cmark parser, and
`HtmlRenderOptions` for converting markdown to HTML. These types should
help make it easier to extend the rendering options while remaining
semver compatible. It should also help with just general ergonomics of
using these functions.
