update config validator to assert flag.unique

Author: DannyBen

lib/bashly/config_validator.rb

@@ -118,6 +118,7 @@ def assert_flag(key, value)
 
       assert_boolean "#{key}.private", value['private']
       assert_boolean "#{key}.repeatable", value['repeatable']
+      assert_boolean "#{key}.unique", value['unique']
       assert_boolean "#{key}.required", value['required']
       assert_array "#{key}.allowed", value['allowed'], of: :string
       assert_array "#{key}.conflicts", value['conflicts'], of: :string