Add g:ale_sign_highlight_linenr (#2678)

* add g:ale_sign_highlight_linenr
* Fix tests. Change option default value
* Rename ale_sign_highlight_linenr to ale_sign_highlight_linenrs
* Default ale_sign_highlight_linenrs to 0

autoload/ale/debugging.vim

@@ -50,6 +50,7 @@ let s:global_variable_list = [
 \    'ale_sign_style_error',
 \    'ale_sign_style_warning',
 \    'ale_sign_warning',
+\    'ale_sign_highlight_linenrs',
 \    'ale_statusline_format',
 \    'ale_type_map',
 \    'ale_use_global_executables',

autoload/ale/sign.vim

@@ -20,6 +20,7 @@ let g:ale_sign_info = get(g:, 'ale_sign_info', g:ale_sign_warning)
 let g:ale_sign_offset = get(g:, 'ale_sign_offset', 1000000)
 " This flag can be set to 1 to keep sign gutter always open
 let g:ale_sign_column_always = get(g:, 'ale_sign_column_always', 0)
+let g:ale_sign_highlight_linenrs = get(g:, 'ale_sign_highlight_linenrs', 0)
 
 if !hlexists('ALEErrorSign')
     highlight link ALEErrorSign error
@@ -82,7 +83,7 @@ execute 'sign define ALEInfoSign text=' . s:EscapeSignText(g:ale_sign_info)
 \   . ' texthl=ALEInfoSign linehl=ALEInfoLine'
 sign define ALEDummySign
 
-if has('nvim-0.3.2')
+if g:ale_sign_highlight_linenrs && has('nvim-0.3.2')
     if !hlexists('ALEErrorSignLineNr')
         highlight link ALEErrorSignLineNr CursorLineNr
     endif