feat: add warning about node version on startup (close #51)

Author: yyx990803

bin/vuepress.js

@@ -1,7 +1,18 @@
 #!/usr/bin/env node
 
-const path = require('path')
 const chalk = require('chalk')
+const semver = require('semver')
+const requiredVersion = require('../package.json').engines.node
+
+if (!semver.satisfies(process.version, requiredVersion)) {
+  console.log(chalk.red(
+    `You are using Node ${process.version}, but VuePress ` +
+    `requires Node ${requiredVersion}.\nPlease upgrade your Node version.`
+  ))
+  process.exit(1)
+}
+
+const path = require('path')
 const { dev, build } = require('../lib')
 
 const program = require('commander')

package.json

@@ -75,6 +75,7 @@
     "prismjs": "^1.13.0",
     "register-service-worker": "^1.2.0",
     "rimraf": "^2.6.2",
+    "semver": "^5.5.0",
     "stylus": "^0.54.5",
     "stylus-loader": "^3.0.2",
     "url-loader": "^1.0.1",

yarn.lock

@@ -5077,7 +5077,7 @@ semver-diff@^2.0.0:
   dependencies:
     semver "^5.0.3"
 
-"semver@2 || 3 || 4 || 5", semver@^5.0.3, semver@^5.1.0, semver@^5.3.0:
+"semver@2 || 3 || 4 || 5", semver@^5.0.3, semver@^5.1.0, semver@^5.3.0, semver@^5.5.0:
   version "5.5.0"
   resolved "https://registry.yarnpkg.com/semver/-/semver-5.5.0.tgz#dc4bbc7a6ca9d916dee5d43516f0092b58f7b8ab"