Add completion.get_trigger_characters as configurable option

lua/cmp/config/default.lua

@@ -13,6 +13,9 @@ return function()
       completeopt = 'menu,menuone,noselect',
       keyword_pattern = [[\%(-\?\d\+\%(\.\d\+\)\?\|\h\w*\%(-\w*\)*\)]],
       keyword_length = 1,
+      get_trigger_characters = function(trigger_characters)
+        return trigger_characters
+      end
     },
 
     snippet = {