feat(microformats): add tag

* http://microformats.org/wiki/rel-tag

themes/typing/layout/_partial/post/tag.ejs

@@ -1,5 +1,5 @@
 <% if (post.tags && post.tags.length) { %>
-  <%- list_tags(post.tags, {
+  <%- listTags(post.tags, {
     show_count: false,
     class: 'article-tag',
     transform: titlecase

themes/typing/scripts/listTags.js

@@ -0,0 +1,72 @@
+/*
+* Modified from the hexo version,
+* https://github.com/hexojs/hexo/blob/master/lib/plugins/helper/list_tags.js
+* to add rel="tag"
+* http://microformats.org/wiki/rel-tag
+*/
+
+'use strict'
+
+function listTagsHelper(tags, options) {
+  if (!options && (!tags || !tags.hasOwnProperty('length'))) {
+    options = tags
+    tags = this.site.tags
+  }
+
+  if (!tags || !tags.length) return ''
+  options = options || {}
+
+  const { style = 'list', transform, separator = ', ', suffix = '' } = options
+  const showCount = options.hasOwnProperty('show_count') ? options.show_count : true
+  const className = options.class || 'tag'
+  const orderby = options.orderby || 'name'
+  const order = options.order || 1
+  let result = ''
+  const self = this
+
+  // Sort the tags
+  tags = tags.sort(orderby, order)
+
+  // Ignore tags with zero posts
+  tags = tags.filter(tag => tag.length)
+
+  // Limit the number of tags
+  if (options.amount) tags = tags.limit(options.amount)
+
+  if (style === 'list') {
+    result += `<ul class="${className}-list">`
+
+    tags.forEach(tag => {
+      result += `<li class="${className}-list-item">`
+
+      result += `<a class="${className}-list-link" href="${self.url_for(tag.path)}${suffix}" rel="tag">`
+      result += transform ? transform(tag.name) : tag.name
+      result += '</a>'
+
+      if (showCount) {
+        result += `<span class="${className}-list-count">${tag.length}</span>`
+      }
+
+      result += '</li>'
+    })
+
+    result += '</ul>'
+  } else {
+    tags.forEach((tag, i) => {
+      if (i) result += separator
+
+      result += `<a class="${className}-link" href="${self.url_for(tag.path)}${suffix}" rel="tag">`
+      result += transform ? transform(tag.name) : tag.name
+
+      if (showCount) {
+        result += `<span class="${className}-count">${tag.length}</span>`
+      }
+
+      result += '</a>'
+    })
+  }
+
+  return result
+}
+
+hexo.extend.helper.register('listTags', listTagsHelper)