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

hugolib/language_content_dir_test.go

@@ -314,7 +314,7 @@ Content.
 	nnSect := nnSite.getPage(page.KindSection, "sect")
 	c.Assert(nnSect, qt.Not(qt.IsNil))
 	c.Assert(len(nnSect.Pages()), qt.Equals, 12)
-	nnHome, _ := nnSite.Info.Home()
+	nnHome := nnSite.Info.Home()
 	c.Assert(nnHome.RelPermalink(), qt.Equals, "/nn/")
 }