title: Filter an array by NOT containing string (swift 2)
body: <p>I am relatively new to swift; I am working on filtering arrays.
  I know how to filter out elements of an array that contain a letter (like so: <code>let filteredList = wordlist.filter { !$0.characters.contains(letter) }</code>), but how do I filter out elements that do not have a letter?
  Here's what I want to accomplish:
  I have a word list in string-array format, i.e. ["thing", "other thing"] (but much longer), and I want to return every element that has a certain letter, filtering out the ones that do not have a certain letter.</p>
  
  <p>Thanks in advance.</p>