Enable 24bit true color support
Enable 24bit true color support

This commit sets the "termguicolors" option to enable support for
24bit true colors (1). This is necessary to use the Nord Vim
"Custom Comment Brightness" (2) theme feature which makes use of HEX
defined colors.

References:
  (1) https://gist.github.com/XVilka/8346728
  (2) https://github.com/arcticicestudio/nord-vim#comment-contrast

GH-37

arcticicestudio/igloo,arcticicestudio/igloo,arcticicestudio/igloo