v0.16.0

• Update cmark-gfm from upstream, including a denial-of-service fix for tables with a large number of autocompleted cells, corrected end_line source positions for single-line and multi-line HTML blocks, and a fix for trailing newlines when rendering inline nodes.
• Add support for front matter (CMARK_OPT_FRONT_MATTER): a --- delimited block at the start of a document is captured as a CMARK_NODE_FRONT_MATTER node. The raw content is available via node.string_content and an optional format hint (e.g. "yaml", "toml") via node.fence_info.
• Allow : in HTML tag names to support XML namespace prefixes (e.g. <svg:circle>, <xhtml:div>).