title: Nim equivalent of Python's list comprehension
body: <p>Since Nim shares a lot of features with Python, i would not be surprised if it implements <a href="http://python-3-patterns-idioms-test.readthedocs.org/en/latest/Comprehensions.html#list-comprehensions">Python's list comprehension</a> too:</p>
  
  <pre><code>string = "Hello 12345 World"
  numbers = [x for x in string if x.isdigit()]
  # ['1', '2', '3', '4', '5']
  </code></pre>
  
  <p>Is this actually possible in Nim? If not, could be implemented with templates/macros ?</p>