Add colorscheme for ALE plugin

colors/gruvbox.vim

@@ -806,6 +806,15 @@ call s:HL('BufTabLineActive', s:fg4, s:bg2)
 call s:HL('BufTabLineHidden', s:bg4, s:bg1)
 call s:HL('BufTabLineFill', s:bg0, s:bg0)
 
+" }}}
+" Asynchronous Lint Engine: {{{
+
+call s:HL('ALEError', s:none, s:none, s:undercurl, s:red)
+call s:HL('ALEWarning', s:none, s:none, s:undercurl, s:yellow)
+
+hi! link ALEErrorSign GruvboxRedSign
+hi! link ALEWarningSign GruvboxYellowSign
+
 " }}}
 
 " Filetype specific -----------------------------------------------------------