scrollbar: allow to customize minimum window width

closes #2531

autoload/airline/extensions/scrollbar.vim

@@ -4,7 +4,8 @@
 scriptencoding utf-8
 
 function! airline#extensions#scrollbar#calculate() abort
-  if winwidth(0) > 200 && get(w:, 'airline_active', 0)
+  if winwidth(0) > get(g:, 'airline#extensions#scrollbar#minwidth', 200)
+      \ && get(w:, 'airline_active', 0)
     let overwrite = 0
     if &encoding ==? 'utf-8' && !get(g:, 'airline_symbols_ascii', 0)
       let [left, right, middle] = [ '|', '|', '█']