Less for Trema i.e. support pre-processing

[marcelstoer]

Add support for pre-processing Trema files to allow for externalization of common text elements. The XML Schema would have to be extended to support something like this:

<trema masterLang="en">
  <token key="email">
    <value lang="en" status="translated">[email protected]</value>
  </token>
  <text key="somekey">
    <value lang="en" status="translated">To reach out email us at $trm{email}.</value>