code: function( chunk, context, bodies, params ){
      if( params && params.cond ){
        var cond = params.cond;
        cond = this.tap(cond, chunk, context);
        // eval expressions with given dust references
        if( eval( cond ) ){
         return chunk.render( bodies.block, context );
        }
        if( bodies['else'] ){
         return chunk.render( bodies['else'], context );
        }
      }
      // no condition
      else {
        _console.log( "No condition given in the if helper!" );
      }
      return chunk;
    }
docs: if helper
  @param cond, either a string literal value or a dust reference
  a string literal value, is enclosed in double quotes, e.g. cond="2>3"
  a dust reference is also enclosed in double quotes, e.g. cond="'{val}'' > 3"
  cond argument should evaluate to a valid javascript expression