Add Markdown diagrams and render hooks for code blocks

You can now create custom hook templates for code blocks, either one for all (`render-codeblock.html`) or for a given code language (e.g. `render-codeblock-go.html`).

We also used this new hook to add support for diagrams in Hugo:

* Goat (Go ASCII Tool) is built-in and enabled by default; just create a fenced code block with the language `goat` and start draw your Ascii diagrams.
* Another popular alternative for diagrams in Markdown, Mermaid (supported by GitHub), can also be implemented with a simple template. See the Hugo documentation for more information.

Updates #7765
Closes #9538
Fixes #9553
Fixes #8520
Fixes #6702
Fixes #9558

common/hugio/writers.go

@@ -18,6 +18,14 @@ import (
 	"io/ioutil"
 )
 
+// As implemented by strings.Builder.
+type FlexiWriter interface {
+	io.Writer
+	io.ByteWriter
+	WriteString(s string) (int, error)
+	WriteRune(r rune) (int, error)
+}
+
 type multiWriteCloser struct {
 	io.Writer
 	closers []io.WriteCloser