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/README.md

@@ -0,0 +1,147 @@
+goldmark-meta
+=========================
+
+goldmark-meta is an extension for the [goldmark](http://github.com/yuin/goldmark) 
+that allows you to define document metadata in YAML format.
+
+Usage
+--------------------
+
+### Installation
+
+```
+go get github.com/yuin/goldmark-meta
+```
+
+### Markdown syntax
+
+YAML metadata block is a leaf block that can not have any markdown element
+as a child.
+
+YAML metadata must start with a **YAML metadata separator**.
+This separator must be at first line of the document.
+
+A **YAML metadata separator** is a line that only `-` is repeated.
+
+YAML metadata must end with a **YAML metadata separator**.
+
+You can define objects as a 1st level item. At deeper level, you can define 
+any kind of YAML element.
+
+Example:
+
+```
+---
+Title: goldmark-meta
+Summary: Add YAML metadata to the document
+Tags:
+    - markdown
+    - goldmark
+---
+
+# Heading 1
+```
+
+
+### Access the metadata
+
+```go
+import (
+    "bytes"
+    "fmt"
+    "github.com/yuin/goldmark"
+    "github.com/yuin/goldmark/extension"
+    "github.com/yuin/goldmark/parser"
+    "github.com/yuin/goldmark-meta"
+)
+
+func main() {
+    markdown := goldmark.New(
+        goldmark.WithExtensions(
+            meta.Meta,
+        ),
+    )
+    source := `---
+Title: goldmark-meta
+Summary: Add YAML metadata to the document
+Tags:
+    - markdown
+    - goldmark
+---
+
+# Hello goldmark-meta
+`
+
+    var buf bytes.Buffer
+    context := parser.NewContext()
+    if err := markdown.Convert([]byte(source), &buf, parser.WithContext(context)); err != nil {
+        panic(err)
+    }
+    metaData := meta.Get(context)
+    title := metaData["Title"]
+    fmt.Print(title)
+}
+```
+
+### Render the metadata as a table
+
+You need to add `extension.TableHTMLRenderer` or the `Table` extension to
+render metadata as a table.
+
+```go
+import (
+    "bytes"
+    "fmt"
+    "github.com/yuin/goldmark"
+    "github.com/yuin/goldmark/extension"
+    "github.com/yuin/goldmark/parser"
+    "github.com/yuin/goldmark/renderer"
+    "github.com/yuin/goldmark/util"
+    "github.com/yuin/goldmark-meta"
+)
+
+func main() {
+    markdown := goldmark.New(
+        goldmark.WithExtensions(
+            meta.New(meta.WithTable()),
+        ),
+        goldmark.WithRendererOptions(
+            renderer.WithNodeRenderers(
+                util.Prioritized(extension.NewTableHTMLRenderer(), 500),
+            ),
+        ),
+    )
+    // OR
+    // markdown := goldmark.New(
+    //     goldmark.WithExtensions(
+    //         meta.New(meta.WithTable()),
+    //         extension.Table,
+    //     ),
+    // )
+    source := `---
+Title: goldmark-meta
+Summary: Add YAML metadata to the document
+Tags:
+    - markdown
+    - goldmark
+---
+
+# Hello goldmark-meta
+`
+
+    var buf bytes.Buffer
+    if err := markdown.Convert([]byte(source), &buf); err != nil {
+        panic(err)
+    }
+    fmt.Print(buf.String())
+}
+```
+
+
+License
+--------------------
+MIT
+
+Author
+--------------------
+Yusuke Inuzuka