title: Vue "npm run build" Ignores vue.config.js File
body: <p>I have a Webpack-templated Vue project, initiated through vue-cli.</p>
  
  <p>I have created a simple '<em>vue.config.js</em>' file stored in the root folder <em>(where package.json is at)</em> containing the following:</p>
  
  <pre><code>// vue.config.js
  module.exports = {
    productionSourceMap: false
  }
  </code></pre>
  
  <p>Though when building the project using "npm run build" it ignores it.</p>
  
  <p>I have tried different configurations to check if the problem is with the file or the setting, and the problem is with the file.</p>
  
  <p>I am using <em>webpack@3.12.0</em>, <em>vue@2.6.11</em>, <em>@vue/cli 4.2.3</em> and <em>npm@6.9.0</em>.</p>