Allow Unicode slugs

examples/full.chapters.json

@@ -7,7 +7,7 @@
     ]
   },
   {
-    "slug": "2",
+    "slug": "2-юникод",
     "title": "Chapter 2",
     "chapters": [
 

examples/full.html

@@ -69,7 +69,7 @@ The Rty
 Chapter 1 paragraph text
 </span>
 </p>
-<h2 id="chapter-2">Chapter 2</h2>
+<h2 id="chapter-2-юникод">Chapter 2</h2>
 <p>
 <span>
 Chapter 2 paragraph text

examples/full.repub

@@ -41,7 +41,7 @@ end
 
 
 
-article.chapter '2', 'Chapter 2' do |chapter|
+article.chapter '2-юникод', 'Chapter 2' do |chapter|
   chapter.paragraph do |paragraph|
     paragraph.text 'Chapter 2 paragraph text'
   end

lib/repubmark.rb

@@ -66,7 +66,7 @@ require_relative 'repubmark/elems/footnote'
 module Repubmark
   FORMATS = %i[chapters gemtext html summary_plain word_count].freeze
 
-  SLUG_RE = /\A\w+(-\w+)*\z/
+  SLUG_RE = /\A[[:word:]]+(-[[:word:]]+)*\z/
 
   UNICODE_SUPS = {
     '0' => '⁰',