Allow inclusion of html content

WebsiteGenerator/Generators/MarkdownProcessor.swift

@@ -79,11 +79,15 @@ struct PageContentGenerator {
         // External pages: ![external](url1, text1; url2, text2, ...)
         let fileAndTitle = markdown.between("(", and: ")")
         let alt = markdown.between("[", and: "]").nonEmpty
-        if alt == "download" {
+        switch alt {
+        case "download":
             return handleDownloadButtons(page: page, content: fileAndTitle)
-        }
+        case "external":
-        if alt == "external" {
             return handleExternalButtons(page: page, content: fileAndTitle)
+        case "html":
+            return handleExternalHTML(page: page, file: fileAndTitle)
+        default:
+            break
         }
 
         let file = fileAndTitle.dropAfterFirst(" ")
@@ -205,4 +209,18 @@ struct PageContentGenerator {
             }
         return factory.html.externalButtons(buttons)
     }
+
+    private func handleExternalHTML(page: Element, file: String) -> String {
+        let url = page.inputFolder.appendingPathComponent(file)
+        guard url.exists else {
+            log.add(error: "File \(file) not found", source: page.path)
+            return ""
+        }
+        do {
+            return try String(contentsOf: url)
+        } catch {
+            log.add(error: "File \(file) could not be read", source: page.path, error: error)
+            return ""
+        }
+    }
 }