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

1 files changed, 10 insertions, 6 deletions

diff --git a/hugolib/site.go b/hugolib/site.go
index 02380a6e7..57821ee93 100644
--- a/hugolib/site.go
+++ b/hugolib/site.go
@@ -30,6 +30,7 @@ import (
 	"strings"
 	"time"
 
+	"github.com/gohugoio/hugo/common/hugio"
 	"github.com/gohugoio/hugo/common/types"
 	"github.com/gohugoio/hugo/modules"
 	"golang.org/x/text/unicode/norm"
@@ -54,12 +55,11 @@ import (
 
 	"github.com/gohugoio/hugo/common/maps"
 
-	"github.com/pkg/errors"
-
 	"github.com/gohugoio/hugo/common/text"
 
 	"github.com/gohugoio/hugo/common/hugo"
 	"github.com/gohugoio/hugo/publisher"
+	"github.com/pkg/errors"
 	_errors "github.com/pkg/errors"
 
 	"github.com/gohugoio/hugo/langs"
@@ -1773,19 +1773,23 @@ var infoOnMissingLayout = map[string]bool{
 	"404": true,
 }
 
-// hookRenderer is the canonical implementation of all hooks.ITEMRenderer,
+// hookRendererTemplate is the canonical implementation of all hooks.ITEMRenderer,
 // where ITEM is the thing being hooked.
-type hookRenderer struct {
+type hookRendererTemplate struct {
 	templateHandler tpl.TemplateHandler
 	identity.SearchProvider
 	templ tpl.Template
 }
 
-func (hr hookRenderer) RenderLink(w io.Writer, ctx hooks.LinkContext) error {
+func (hr hookRendererTemplate) RenderLink(w io.Writer, ctx hooks.LinkContext) error {
+	return hr.templateHandler.Execute(hr.templ, w, ctx)
+}
+
+func (hr hookRendererTemplate) RenderHeading(w io.Writer, ctx hooks.HeadingContext) error {
 	return hr.templateHandler.Execute(hr.templ, w, ctx)
 }
 
-func (hr hookRenderer) RenderHeading(w io.Writer, ctx hooks.HeadingContext) error {
+func (hr hookRendererTemplate) RenderCodeblock(w hugio.FlexiWriter, ctx hooks.CodeblockContext) error {
 	return hr.templateHandler.Execute(hr.templ, w, ctx)
 }