Port the Python build script over to JS

`npm run build` now builds both the site.json file and the React app.
Two new run scripts are available:

* build_site - just build the site.json file
* build_code - build the React app

scripts/build_site.js

@@ -0,0 +1,77 @@
+#!/usr/bin/env node
+
+'use strict';
+const fs = require('mz/fs');
+const path = require('path');
+const yaml = require('js-yaml');
+
+
+async function build() {
+  let site = {
+    pages: [],
+    posts: [],
+    tags: [],
+  };
+
+  site.pages = await fs.readdir('_pages')
+    .then(files => (
+      Promise.all(files.map(f => fs.readFile(path.join('_pages', f), 'utf8')))
+    ))
+    .then(files => {
+      return files.map(contents => {
+        let [_, meta, body] = contents.split("---\n");
+        let page = yaml.safeLoad(meta);
+
+        if (page.parent === undefined) {
+          page.parent = null;
+        }
+
+        page.body = body.trim();
+        return page;
+      })
+    })
+    .catch(err => console.log('Failure while fetching pages:', err));
+
+  [site.posts, site.tags] = await fs.readdir('_posts')
+    .then(files => (
+      Promise.all(files.map(f => fs.readFile(path.join('_posts', f), 'utf8')))
+    ))
+    .then(files => {
+      let tags = new Set();
+      let posts = files.map(contents => {
+        let parts = contents.split("---\n");
+        let post = yaml.safeLoad(parts[0]);
+
+        if (parts.length === 2) {
+          post.blurb = parts[1];
+          post.body = post.blurb;
+        } else if (parts.length === 3) {
+          post.blurb = parts[1];
+          post.body = parts.slice(1).join("\n");
+        }
+
+        if (post.tags === undefined) {
+          post.tags = [];
+        }
+
+        post.tags.forEach(tag => {
+          tags.add(tag);
+        });
+
+        return post;
+      })
+
+      return [posts, tags];
+    })
+    .catch(err => console.log('Failure while fetching posts:', err));
+
+  site.pages.sort((a, b) => a.title.localeCompare(b.title));
+  // sort posts newest to oldest
+  site.posts.sort((a, b) => (b.date - a.date));
+  site.tags = Array.from(site.tags.values());
+
+  fs.writeFile('public/site.json', JSON.stringify(site))
+    .catch(err => console.log('Unable to write site.json:', err));
+}
+
+build();