cache()
  Cache return value of a scanner.
  
  First call of this scanner caches return value for a scanner included into
  and return it's value. Next calls returns stored values.
  
  Usage: cache('VAR',scanners...)
  
  Where: 'VAR' is a string, which defines variable name to cache return value
  
  Example: cache('var1', scanner1()) # to store and return scanner1()'s value
           cache('var1') # to only return previously stored value
  
  New in version 0.7.0.