title: JWTAuth authentication with username instead of email
body: <p>Is there any way to use in laravel <a href="http://ift.tt/1GFm4Yr" rel="noreferrer">jwt-auth</a> the authentication using <strong>username</strong> (or any custom column), instead of an <strong>email</strong>?</p>
  
  <p>This is the existing code</p>
  
  <pre><code>public function authenticate(Request $request)
  {
      $credentials = $request-&gt;only('email', 'password');
  
      try {
          // verify the credentials and create a token for the user
          if (! $token = JWTAuth::attempt($credentials)) {
              return response()-&gt;json(['error' =&gt; 'invalid_credentials'], 401);
          }
      } catch (JWTException $e) {
          // something went wrong
          return response()-&gt;json(['error' =&gt; 'could_not_create_token'], 500);
      }
  
      // if no errors are encountered we can return a JWT
      return response()-&gt;json(compact('token'));
  }
  </code></pre>
  
  <p>I want to do Something like this</p>
  
  <pre><code>$credentials = $request-&gt;only('username', 'password');
  </code></pre>
  
  <p>One way is to put <strong>usernames</strong> in <strong>email</strong> column but that is a bad solution.</p>