Plugin support for vim-startify (#176)

Added custom highlight groups of the `vim-startify` (1) plugin to adapt
to Nord's style.

References:
  (1) https://github.com/mhinz/vim-startify

Resolves GH-159

colors/nord.vim

@@ -609,6 +609,17 @@ call s:hi("plugDeleted", s:nord11_gui, "", "", s:nord11_term, "", "")
 " > kshenoy/vim-signature
 call s:hi("SignatureMarkText", s:nord8_gui, "", s:nord8_term, "", "", "")
 
+" vim-startify
+" > mhinz/vim-startify
+call s:hi("StartifyFile", s:nord6_gui, "", s:nord6_term, "", "", "")
+call s:hi("StartifyFooter", s:nord7_gui, "", s:nord7_term, "", "", "")
+call s:hi("StartifyHeader", s:nord8_gui, "", s:nord8_term, "", "", "")
+call s:hi("StartifyNumber", s:nord7_gui, "", s:nord7_term, "", "", "")
+call s:hi("StartifyPath", s:nord8_gui, "", s:nord8_term, "", "", "")
+hi! link StartifyBracket Delimiter
+hi! link StartifySlash Normal
+hi! link StartifySpecial Comment
+
 "+--- Languages ---+
 " Haskell
 " > neovimhaskell/haskell-vim