Add option to use custom header code

WebsiteGenerator/Content/Page+Metadata.swift

@@ -20,6 +20,14 @@ extension Page {
 
         let requiredFiles: [String]
 
+        /**
+         Indicate that no header should be generated automatically.
+
+         This option assumes that custom header code is present in the page source files
+         - Note: If not specified, this property defaults to `false`.
+         */
+        let useCustomHeader: Bool
+
         #warning("Add files for which errors are ignored when missing")
     }
 }
@@ -36,7 +44,8 @@ extension Page.Metadata: Metadata {
             isDraft: true,
             sortIndex: 0,
             languages: [.initial],
-            requiredFiles: [])
+            requiredFiles: [],
+            useCustomHeader: false)
     }
 }
 
@@ -54,6 +63,7 @@ extension Page.Metadata: Codable {
         case sortIndex
         case languages
         case requiredFiles
+        case useCustomHeader
     }
 
     func encode(to encoder: Encoder) throws {
@@ -71,6 +81,9 @@ extension Page.Metadata: Codable {
         if !requiredFiles.isEmpty {
             try container.encode(requiredFiles, forKey: .requiredFiles)
         }
+        if useCustomHeader {
+            try container.encode(true, forKey: .useCustomHeader)
+        }
     }
 
     init(from decoder: Decoder) throws {
@@ -89,5 +102,6 @@ extension Page.Metadata: Codable {
             self.endDate = nil
         }
         self.requiredFiles = try container.decodeIfPresent([String].self, forKey: .requiredFiles) ?? []
+        self.useCustomHeader = try container.decodeIfPresent(Bool.self, forKey: .useCustomHeader) ?? false
     }
 }