Comment Color Brightness

-> Please see nordtheme/nord#94 for all
   details about this design change decision.

Increased the comment color (`nord3`) brightness by 10% from a lightness
level of ~35% to ~45%.

This change also deprecates the comment contrast (1) configuration.
It is not necessary anymore for users to increase the brightness on
their own when the default color has been increased by default.
A deprecation warning will be shown to notify all users who have set a
custom value for the `g:nord_comment_brightness` configuration variable.

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

GH-145

colors/nord.vim

@@ -20,6 +20,7 @@ let s:nord0_gui = "#2E3440"
 let s:nord1_gui = "#3B4252"
 let s:nord2_gui = "#434C5E"
 let s:nord3_gui = "#4C566A"
+let s:nord3_gui_bright = "#616E88"
 let s:nord4_gui = "#D8DEE9"
 let s:nord5_gui = "#E5E9F0"
 let s:nord6_gui = "#ECEFF4"
@@ -100,8 +101,17 @@ if !exists('g:nord_uniform_status_lines')
   let g:nord_uniform_status_lines = 0
 endif
 
-if !exists("g:nord_comment_brightness")
-  let g:nord_comment_brightness = 0
+function! s:logWarning(msg)
+  echohl WarningMsg
+  echomsg 'nord: warning: ' . a:msg
+  echohl None
+endfunction
+
+if exists("g:nord_comment_brightness")
+  call s:logWarning('Variable g:nord_comment_brightness has been deprecated and will be removed in version 1.0.0!' .
+                   \' The comment color brightness has been increased by 10% by default.' .
+                   \' Please see https://github.com/arcticicestudio/nord-vim/issues/145 for more details.')
+  let g:nord_comment_brightness = 10
 endif
 
 if !exists("g:nord_uniform_diff_background")
@@ -245,7 +255,7 @@ call s:hi("VertSplit", s:nord2_gui, s:nord1_gui, s:nord3_term, s:nord1_term, "NO
 "+----------------------+
 call s:hi("Boolean", s:nord9_gui, "", s:nord9_term, "", "", "")
 call s:hi("Character", s:nord14_gui, "", s:nord14_term, "", "", "")
-call s:hi("Comment", s:nord3_gui_brightened[g:nord_comment_brightness], "", s:nord3_term, "", s:italicize_comments, "")
+call s:hi("Comment", s:nord3_gui_bright, "", s:nord3_term, "", s:italicize_comments, "")
 call s:hi("Conditional", s:nord9_gui, "", s:nord9_term, "", "", "")
 call s:hi("Constant", s:nord4_gui, "", "NONE", "", "", "")
 call s:hi("Define", s:nord9_gui, "", s:nord9_term, "", "", "")
@@ -471,7 +481,7 @@ hi! link vimFunction Function
 hi! link vimUserFunc Function
 
 call s:hi("xmlAttrib", s:nord7_gui, "", s:nord7_term, "", "", "")
-call s:hi("xmlCdataStart", s:nord3_gui, "", s:nord3_term, "", "bold", "")
+call s:hi("xmlCdataStart", s:nord3_gui_bright, "", s:nord3_term, "", "bold", "")
 call s:hi("xmlNamespace", s:nord7_gui, "", s:nord7_term, "", "", "")
 hi! link xmlAttribPunct Delimiter
 hi! link xmlCdata Comment