add highlight from brew and hl command

hl command accepts syntax type and optionally a filename as arguments.
If a filename is given, it will syntax highlight the file and place the
contents on the clipboard. If no file is given, it will use the contents
of the clipboard.

Brewfile

@@ -16,6 +16,7 @@ install macvim --override-system-vim
 install reattach-to-user-namespace
 install tmux
 install zsh
+install highlight
 
 # gitsh
 tap thoughtbot/formulae