title: Webpack ts-loader : change tsconfig filename
body: <p>I have 2 tsconfigs, one for my dev build and one for my prod build.<br>
  I choose the tsconfig with the <code>-p</code> flag :<br>
  <code>tsc -p dev.tsconfig.json</code></p>
  
  <p>Ts-loader is looking for a tsconfig.json file. How can I specify another filename with the ts-loader?</p>
  
  <pre><code>module.exports = {
      entry : __dirname + "/src/app/main.ts",
      output : {
          path : __dirname + "/dist",
          filename : "bundle.js"
      },
     resolve : {
       extensions : ["", ".webpack.js", ".web.js", ".js", ".ts"]
     },
     module: {
          loaders: [
              { test: /\.ts?$/, loader: "ts" }
          ]
      }
  };
  </code></pre>