Add a prefix to footnote IDs.

This avoids a conflict with multiple blog posts rendering in the same stream.

src/context/render_context.rs

@@ -10,6 +10,13 @@ pub(crate) struct RenderContext<'intermediate> {
     // TODO: Perhaps rename to output_root_directory.
     pub(crate) output_directory: &'intermediate Path,
     pub(crate) output_file: &'intermediate Path,
+
+    /// An optional string that gets added to IDs in HTML.
+    ///
+    /// This is useful for cases where you may have conflicting HTML
+    /// IDs, for example, multiple blog posts with footnotes in a blog
+    /// stream.
+    pub(crate) id_addition: Option<&'intermediate str>,
 }
 
 impl<'intermediate> RenderContext<'intermediate> {
@@ -17,11 +24,13 @@ impl<'intermediate> RenderContext<'intermediate> {
         config: &'intermediate Config,
         output_directory: &'intermediate Path,
         output_file: &'intermediate Path,
+        id_addition: Option<&'intermediate str>,
     ) -> Result<RenderContext<'intermediate>, CustomError> {
         Ok(RenderContext {
             config,
             output_directory,
             output_file,
+            id_addition,
         })
     }
 }