Implement links in section headers.

This project already had a transitive dependency on regex; let's use it.

This isn't the most efficient solution, but it should be fine. It ends
up doing five full scans of the text. There's probably an easier way but
I'm mostly just trying to get this to work for now.

This also implements the same algorithm that rustdoc does for generating
the name for the link.

Fixes #204

Cargo.toml

@@ -24,6 +24,7 @@ log = "0.3"
 env_logger = "0.3"
 toml = { version = "0.2", features = ["serde"] }
 open = "1.1"
+regex = "0.1.80"
 
 # Watch feature
 notify = { version = "3.0", optional = true }

src/lib.rs

@@ -74,6 +74,7 @@ extern crate serde;
 extern crate serde_json;
 extern crate handlebars;
 extern crate pulldown_cmark;
+extern crate regex;
 
 #[macro_use] extern crate log;
 pub mod book;

src/renderer/html_handlebars/hbs_renderer.rs

@@ -3,7 +3,9 @@ use renderer::Renderer;
 use book::MDBook;
 use book::bookitem::BookItem;
 use {utils, theme};
+use regex::{Regex, Captures};
 
+use std::ascii::AsciiExt;
 use std::path::{Path, PathBuf};
 use std::fs::{self, File};
 use std::error::Error;
@@ -92,6 +94,9 @@ impl Renderer for HtmlHandlebars {
                         debug!("[*]: Render template");
                         let rendered = try!(handlebars.render("index", &data));
                         
+                        // create links for headers
+                        let rendered = build_header_links(rendered);
+
                         // Write to file
                         let filename = Path::new(&ch.path).with_extension("html");
                         info!("[*] Creating {:?} ✓", filename.display());
@@ -208,3 +213,37 @@ fn make_data(book: &MDBook) -> Result<serde_json::Map<String, serde_json::Value>
     debug!("[*]: JSON constructed");
     Ok(data)
 }
+
+fn build_header_links(mut html: String) -> String {
+    for header in &["h1", "h2", "h3", "h4", "h5"] {
+        let regex = Regex::new(&format!("<{h}>(.*?)</{h}>", h=header)).unwrap();
+
+        html = regex.replace_all(&html, |caps: &Captures| {
+            let text = &caps[1];
+            let mut id = text.to_string();
+            let repl_sub = vec!["<em>", "</em>", "<code>", "</code>",
+                                "<strong>", "</strong>",
+                                "&lt;", "&gt;", "&amp;", "&#39;", "&quot;"];
+            for sub in repl_sub {
+                id = id.replace(sub, "");
+            }
+            let id = id.chars().filter_map(|c| {
+                if c.is_alphanumeric() || c == '-' || c == '_' {
+                    if c.is_ascii() {
+                        Some(c.to_ascii_lowercase())
+                    } else {
+                        Some(c)
+                    }
+                } else if c.is_whitespace() && c.is_ascii() {
+                    Some('-')
+                } else {
+                    None
+                }
+            }).collect::<String>();
+
+            format!("<a class=\"header\" href=\"#{id}\" name=\"{id}\"><{h}>{text}</{h}></a>", h=header, id=id, text=text)
+        });
+    }
+
+    html
+}