RIIR update_lints: Replace lint count in README.md

This allows the usage of `util/dev update_lints` which will write the
new lint_count to the `README.md`.

clippy_dev/src/main.rs

@@ -32,6 +32,8 @@ fn main() {
     if let Some(matches) = matches.subcommand_matches("update_lints") {
         if matches.is_present("print-only") {
             print_lints();
+        } else {
+            update_lints();
         }
     }
 }
@@ -55,3 +57,21 @@ fn print_lints() {
 
     println!("there are {} lints", lint_count);
 }
+
+fn update_lints() {
+    let lint_list = gather_all();
+    let usable_lints: Vec<Lint> = Lint::usable_lints(lint_list).collect();
+    let lint_count = usable_lints.len();
+
+    replace_region_in_file(
+        "../README.md",
+        r#"\[There are \d+ lints included in this crate!\]\(https://rust-lang-nursery.github.io/rust-clippy/master/index.html\)"#,
+        "",
+        true,
+        || {
+            vec![
+                format!("[There are {} lints included in this crate!](https://rust-lang-nursery.github.io/rust-clippy/master/index.html)", lint_count)
+            ]
+        }
+    );
+}