title: nvim in macOS Terminal breaks on displaying termguicolors
body: <p>I have been using Neovim in iTerm2 so far and it displays my Vim syntax just fine. 
  I started to use the good ol' Mac OS Terminal and it cannot display my Vim syntax colors after using <code>set termguicolors</code>.</p>
  
  <p>On the left side I have iTerm and on the right side is the OSX Terminal with the standard nvim highlighting.
  <a href="https://i.stack.imgur.com/iybLZ.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/iybLZ.png" alt="enter image description here"></a></p>
  
  <p>After turning on <code>termguicolors</code> it looks like this: (iTerm works as expected and OSX Terminal presents a gray mess)
  <a href="https://i.stack.imgur.com/NzGUt.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/NzGUt.png" alt="enter image description here"></a></p>
  
  <p>I would like to understand why this happens.</p>