fix: handle headers that start with numbers (fix #121)

lib/markdown/slugify.js

@@ -15,6 +15,8 @@ module.exports = function slugify (str) {
     .replace(/\-{2,}/g, '-')
     // Remove prefixing and trailing separtors
     .replace(/^\-+|\-+$/g, '')
+    // ensure it doesn't start with a number (#121)
+    .replace(/^(\d)/, '_$1')
     // lowercase
     .toLowerCase()
 }