title: Case insensitive option with Regex.compile/2
body: <p>I'm attempting to build a caseless regex binary using <a href="https://hexdocs.pm/elixir/Regex.html#compile/2" rel="nofollow noreferrer"><code>Regex.compile/2</code></a> but can't seem to find an example on how the option should be set.</p>
  
  <pre><code>Regex.compile("^(foo):?", :caseless)
  ** (FunctionClauseError) no function clause matching in Regex.compile/3
  
  The following arguments were given to Regex.compile/3:
  
      # 1
      "^(foo):?"
  
      # 2
      :caseless
  
      # 3
      "8.41 2017-07-05"
  
  (elixir) lib/regex.ex:140: Regex.compile/3
  </code></pre>