feat: copy static files for production build

docs/hello.md

@@ -4,3 +4,5 @@ title: Hello, World !
 ---
 
 Hi, Endilie here :)
+
+![sakura](/img/sakura.png)

lib/commands/build.js

@@ -2,6 +2,8 @@ const webpackNiceLog = require('webpack-nicelog');
 const webpack = require('webpack');
 const path = require('path');
 const chalk = require('chalk');
+const fs = require('fs-extra');
+const globby = require('globby');
 const load = require('../load');
 const createProdConfig = require('../webpack/prod');
 
@@ -51,6 +53,20 @@ module.exports = async function build(siteDir, cliOptions = {}) {
   await compile(config);
 
   const {outDir} = props;
+
+  // copy static files
+  const staticDir = path.resolve(siteDir, 'static');
+  const staticFiles = await globby(['**'], {
+    cwd: staticDir
+  });
+  await Promise.all(
+    staticFiles.map(async source => {
+      const fromPath = path.resolve(staticDir, source);
+      const toPath = path.resolve(outDir, source);
+      return fs.copy(fromPath, toPath);
+    })
+  );
+
   const relativeDir = path.relative(process.cwd(), outDir);
   console.log(
     `\n${chalk.green('Success!')} Generated static files in ${chalk.cyan(