Implement matching config.

Fix #796

lua/cmp/config/default.lua

@@ -88,6 +88,12 @@ return function()
       end,
     },
 
+    matching = {
+      disallow_fuzzy_matching = false,
+      disallow_partial_matching = false,
+      disallow_prefix_unmatching = false,
+    },
+
     sorting = {
       priority_weight = 2,
       comparators = {