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

markup/converter/converter.go

@@ -21,6 +21,7 @@ import (
 	"github.com/gohugoio/hugo/config"
 	"github.com/gohugoio/hugo/identity"
 	"github.com/gohugoio/hugo/markup/converter/hooks"
+	"github.com/gohugoio/hugo/markup/highlight"
 	"github.com/gohugoio/hugo/markup/markup_config"
 	"github.com/gohugoio/hugo/markup/tableofcontents"
 	"github.com/spf13/afero"
@@ -34,7 +35,7 @@ type ProviderConfig struct {
 	ContentFs afero.Fs
 	Logger    loggers.Logger
 	Exec      *hexec.Exec
-	Highlight func(code, lang, optsStr string) (string, error)
+	highlight.Highlighter
 }
 
 // ProviderProvider creates converter providers.
@@ -127,9 +128,10 @@ type DocumentContext struct {
 
 // RenderContext holds contextual information about the content to render.
 type RenderContext struct {
-	Src         []byte
-	RenderTOC   bool
-	RenderHooks hooks.Renderers
+	Src       []byte
+	RenderTOC bool
+
+	GetRenderer hooks.GetRendererFunc
 }
 
 var FeatureRenderHooks = identity.NewPathIdentity("markup", "renderingHooks")