Fix header links

Header fragment links now use "id" attribute instead of the depreciated
"name" attribute.

Similar headers are given numbered ids to avoid id collisions.
For instance, if there are three headers named "Example", their ids
would be "#example", "#example-1", and "#example-2" respectively.

src/renderer/html_handlebars/hbs_renderer.rs

@@ -11,6 +11,7 @@ use std::fs::{self, File};
 use std::error::Error;
 use std::io::{self, Read};
 use std::collections::BTreeMap;
+use std::collections::HashMap;
 
 use handlebars::Handlebars;
 
@@ -229,6 +230,7 @@ fn make_data(book: &MDBook) -> Result<serde_json::Map<String, serde_json::Value>
 
 fn build_header_links(html: String, filename: &str) -> String {
     let regex = Regex::new(r"<h(\d)>(.*?)</h\d>").unwrap();
+    let mut id_counter = HashMap::new();
 
     regex.replace_all(&html, |caps: &Captures| {
         let level = &caps[1];
@@ -254,7 +256,16 @@ fn build_header_links(html: String, filename: &str) -> String {
             }
         }).collect::<String>();
 
-        format!("<a class=\"header\" href=\"{filename}#{id}\" name=\"{id}\"><h{level}>{text}</h{level}></a>",
+        let id_count = *id_counter.get(&id).unwrap_or(&0);
+        id_counter.insert(id.clone(), id_count + 1);
+
+        let id = if id_count > 0 {
+            format!("{}-{}", id, id_count)
+        } else {
+            id
+        };
+
+        format!("<a class=\"header\" href=\"{filename}#{id}\" id=\"{id}\"><h{level}>{text}</h{level}></a>",
             level=level, id=id, text=text, filename=filename)
     }).into_owned()
 }