Use absolute links in remark toc (gatsbyjs#2856)

* Avoid assuming default tabWidth in user Prettier config

* Add slugs to markdown TOC

* Try to make TOC prefix-paths aware

* Correctly add prefix paths to links in Remark TOC

* Fix linting and use correct pathPrefix variable

.prettierrc

@@ -1,4 +1,5 @@
 {
-  "trailingComma": "es5",
-  "semi": false
+    "trailingComma": "es5",
+    "semi": false,
+    "tabWidth": 2
 }

packages/gatsby-transformer-remark/src/extend-node-type.js

@@ -182,8 +182,23 @@ module.exports = (
       } else {
         const ast = await getAST(markdownNode)
         const tocAst = mdastToToc(ast)
+
         let toc
         if (tocAst.map) {
+          const addSlugToUrl = function(node) {
+            if (node.url) {
+              node.url = [pathPrefix, markdownNode.fields.slug, node.url]
+                .join(`/`)
+                .replace(/\/\//g, `/`)
+            }
+            if (node.children) {
+              node.children = node.children.map(node => addSlugToUrl(node))
+            }
+
+            return node
+          }
+          tocAst.map = addSlugToUrl(tocAst.map)
+
           toc = hastToHTML(toHAST(tocAst.map))
         } else {
           toc = ``