next init: Create supplied directory if it does not exist. (#136)

* next init: Create supplied directory if it does not exist.

* next init: use mkdirp to init into nested dirs.

Picked mkdirp-then out of many alternatives. Same author & similar
dependencies to mz.

Now next init a/b/c works.

Related https://github.com/zeit/next.js/pull/136#discussion_r85543040

bin/next-init

@@ -2,6 +2,7 @@
 import { resolve, join, basename } from 'path'
 import parseArgs from 'minimist'
 import { exists, writeFile, mkdir } from 'mz/fs'
+import mkdirp from 'mkdirp-then'
 
 const argv = parseArgs(process.argv.slice(2), {
   alias: {
@@ -17,9 +18,13 @@ if (basename(dir) === 'pages') {
   process.exit(1)
 }
 
-exists(join(dir, 'package.json'))
+exists(dir)
 .then(async present => {
   if (!present) {
+    await mkdirp(dir)
+  }
+
+  if (!await exists(join(dir, 'package.json'))) {
     await writeFile(join(dir, 'package.json'), basePackage.replace(/my-app/g, basename(dir)))
   }
 

package.json

@@ -58,6 +58,7 @@
     "json-loader": "0.5.4",
     "loader-utils": "0.2.16",
     "minimist": "1.2.0",
+    "mkdirp-then": "1.2.0",
     "mz": "2.4.0",
     "path-match": "1.2.4",
     "react": "15.3.2",