Add data property to chapter struct

This data property makes it possible to share data
between pre-processors, or between pre-processors
and the renderer, including the "html" renderer
which exposes the chapter data as "data" in the
.hbs files.

examples/nop-preprocessor.rs

@@ -138,7 +138,8 @@ mod nop_lib {
                                 "sub_items": [],
                                 "path": "chapter_1.md",
                                 "source_path": "chapter_1.md",
-                                "parent_names": []
+                                "parent_names": [],
+                                "data": {}
                             }
                         }
                     ],

src/book/book.rs

@@ -165,6 +165,9 @@ pub struct Chapter {
     pub source_path: Option<PathBuf>,
     /// An ordered list of the names of each chapter above this one in the hierarchy.
     pub parent_names: Vec<String>,
+    /// Data that can be shared between preprocessors, accessed in the renderer
+    /// or accessed in the theme's `.hbs` files
+    pub data: serde_json::Map<String, serde_json::Value>,
 }
 
 impl Chapter {
@@ -444,6 +447,7 @@ And here is some \
             source_path: Some(PathBuf::from("second.md")),
             parent_names: vec![String::from("Chapter 1")],
             sub_items: Vec::new(),
+            data: serde_json::Map::new(),
         };
         let should_be = BookItem::Chapter(Chapter {
             name: String::from("Chapter 1"),
@@ -457,6 +461,7 @@ And here is some \
                 BookItem::Separator,
                 BookItem::Chapter(nested),
             ],
+            data: serde_json::Map::new(),
         });
 
         let got = load_summary_item(&SummaryItem::Link(root), temp.path(), Vec::new()).unwrap();
@@ -533,6 +538,7 @@ And here is some \
                             Vec::new(),
                         )),
                     ],
+                    data: serde_json::Map::new(),
                 }),
                 BookItem::Separator,
             ],
@@ -586,6 +592,7 @@ And here is some \
                             Vec::new(),
                         )),
                     ],
+                    data: serde_json::Map::new(),
                 }),
                 BookItem::Separator,
             ],

src/renderer/html_handlebars/hbs_renderer.rs

@@ -105,6 +105,7 @@ impl HtmlHandlebars {
             ctx.data
                 .insert("section".to_owned(), json!(section.to_string()));
         }
+        ctx.data.insert("data".to_owned(), json!(ch.data));
 
         // Render the handlebars template with the data
         debug!("Render template");