Allow book extensions to contribute filter params

Author: dragonstyle

src/project/types/book/book-shared.ts

@@ -6,6 +6,7 @@
 */
 
 import { RenderedFile } from "../../../command/render/types.ts";
+import { PandocOptions, RenderedFile } from "../../../command/render/types.ts";
 import { kTitle } from "../../../config/constants.ts";
 import { Format } from "../../../config/types.ts";
 import { parsePandocTitle } from "../../../core/pandoc/pandoc-partition.ts";
@@ -41,6 +42,8 @@ export interface BookExtension {
   // bool extensions are single file by default but can elect to be multi file
   multiFile?: boolean;
 
+  filterParams?: (options: PandocOptions) => Record<string, unknown>;
+
   formatOutputDirectory?: (format: Format) => string;
 
   // book extensions can modify the format before render

src/project/types/book/book.ts

@@ -136,12 +136,18 @@ export const bookProjectType: ProjectType = {
   },
 
   filterParams: (options: PandocOptions) => {
+    const bookExt = options.format.extensions?.book as BookExtension;
+    const filterParams = bookExt.filterParams
+      ? bookExt.filterParams(options)
+      : {};
     if (isMultiFileBookFormat(options.format)) {
       return {
+        ...filterParams,
         [kCrossrefResolveRefs]: false,
       };
     } else {
       return {
+        ...filterParams,
         [kSingleFileBook]: true,
       };
     }