title: Get expression values in JEXL
body: <p>Having the following JEXL expression:</p>
  
  <pre><code>String expression = "myVar &gt;= 12345 &amp;&amp; mySecondVar &lt;= 56789";
  </code></pre>
  
  <p>I can call createScript and getVariables to get myVar and mySecondVar as values, such as:</p>
  
  <pre><code>Set&lt;List&lt;String&gt;&gt; expressionVars = JEXL.createScript(expression).getVariables();
  </code></pre>
  
  <p>What I would like to know is, if given the same expression, I could call some other method that would return the values for those variables. Reason being is that I would like to validate the input of some of those values. I checked the docs and played around with JexlScript class but can't find an elegant way of doing it.
  As JEXL is already doing the work of parsing my expression, it would be awesome to be able to retrieve this info, and not having to manually parse my expression to get this values.</p>
  
  <p>Something in the line of <code>script.getValue("myVar");</code> returning <code>12345</code></p>