Add configuration options for pandoc

Current options are:

  markup:
    pandoc:
      filters:
        - list
        - of
        - filters
      extensions:
        - list
        - of
        - extensions
      extraArgs:
        - --extra-arguments
        - --one-per-line

Generalize some Pandoc options.

Support configuring a bibliography in markup config

Anonymous Update

[pandoc] Allow page parameters to override site parameters.

This allows specifying things like this in the page frontmatter:

    ---
    title: Something
    bibliography:
      source: mybibliography.bib
    pandoc:
      filter:
        - make-diagrams.lua
    ...

These options are local to the page. Specifying the same under `markup`
in the site configuration applies those settings to all pages.

Paths (filters, bibliography, citation style) are resolved relative to
the page, site, and the `static` folder.

[pandoc] Support metadata

Support specifying Pandoc metadata in the site configuration and page
configuration using the following syntax:

Site (in `config.yaml`):

    ```yaml
    markup:
        pandoc:
                metadata:
                        link-citations: true
    ```

Or in frontmatter:

    ```yaml
    ---
    pandoc:
        metadata:
                link-citations: true
    ...
    ```

[pandoc] Simplify path management.

No need for any fancy path lookup gymnastics. `pandoc`'s
`--resource-path` option does the legwork of locating resources on
multiple directories.

[pandoc] Don't use x != "" to denote failure.

markup/markup_config/config.go

@@ -111,5 +111,6 @@ var Default = Config{
 	Bibliography:    bibliography.Default,
 
 	Goldmark:    goldmark_config.Default,
+	Pandoc:      pandoc_config.Default,
 	AsciidocExt: asciidocext_config.Default,
 }

markup/pandoc/convert.go

@@ -15,6 +15,7 @@
 package pandoc
 
 import (
+	"errors"
 	"strings"
 
 	"github.com/gohugoio/hugo/common/hexec"
@@ -22,7 +23,6 @@ import (
 	"github.com/mitchellh/mapstructure"
 
 	"github.com/gohugoio/hugo/identity"
-
 	"github.com/gohugoio/hugo/markup/bibliography"
 	"github.com/gohugoio/hugo/markup/converter"
 	"github.com/gohugoio/hugo/markup/internal"
@@ -64,7 +64,7 @@ type pandocConverter struct {
 }
 
 func (c *pandocConverter) Convert(ctx converter.RenderContext) (converter.ResultRender, error) {
-	b, err := c.getPandocContent(ctx.Src, c.ctx)
+	b, err := c.getPandocContent(ctx.Src)
 	if err != nil {
 		return nil, err
 	}
@@ -76,17 +76,14 @@ func (c *pandocConverter) Supports(feature identity.Identity) bool {
 }
 
 // getPandocContent calls pandoc as an external helper to convert pandoc markdown to HTML.
-func (c *pandocConverter) getPandocContent(src []byte) []byte {
-	logger := c.cfg.Logger
+func (c *pandocConverter) getPandocContent(src []byte) ([]byte, error) {
 	pandocPath, pandocFound := getPandocBinaryName()
 	if !pandocFound {
-		logger.Println("pandoc not found in $PATH: Please install.\n",
-			"                 Leaving pandoc content unrendered.")
-		return src
+		return nil, errors.New("pandoc not found in $PATH: Please install.")
 	}
 
-	var pandocConfig pandoc_config.Config = c.cfg.MarkupConfig.Pandoc
-	var bibConfig bibliography.Config = c.cfg.MarkupConfig.Bibliography
+	var pandocConfig pandoc_config.Config = c.cfg.MarkupConfig().Pandoc
+	var bibConfig bibliography.Config = c.cfg.MarkupConfig().Bibliography
 
 	if pageParameters, ok := c.docCtx.Document.(paramer); ok {
 		if bibParam, err := pageParameters.Param("bibliography"); err == nil {
@@ -111,7 +108,7 @@ func (c *pandocConverter) getPandocContent(src []byte) []byte {
 	arguments = append(arguments, "--resource-path", resourcePath)
 
 	renderedContent, _ := internal.ExternallyRenderContent(c.cfg, c.docCtx, src, pandocPath, arguments)
-	return renderedContent
+	return renderedContent, nil
 }
 
 const pandocBinary = "pandoc"

markup/pandoc/pandoc_config/pandoc.go

@@ -69,6 +69,12 @@ type Config struct {
 	ExtraArgs []string
 }
 
+var Default = Config{
+	InputFormat:   "markdown",
+	UseLegacyHtml: false,
+	UseMathjax:    true,
+}
+
 func (c *Config) getInputArg() string {
 	var b strings.Builder
 	b.WriteString("--from=")