Handle yaml frontmatter (#11016)

Add goldmark-meta to render yaml frontmatter as a table

Fix #5377

Signed-off-by: Andrew Thornton <art27@cantab.net>

vendor/github.com/yuin/goldmark-meta/.gitignore

@@ -0,0 +1,13 @@
+# Binaries for programs and plugins
+*.exe
+*.exe~
+*.dll
+*.so
+*.dylib
+
+# Test binary, build with `go test -c`
+*.test
+*.pprof
+
+# Output of the go coverage tool, specifically when used with LiteIDE
+*.out