def set_from_env(*keys, &block)
      assert_keys_with_block(convert_to_keys(keys), block)
      key = flatten_keys(keys)
      env_key = block.nil? ? key : block.()
      env_key = to_env_key(env_key)
      @envs[key.to_s.downcase] = env_key
    end
def set_from_env(*keys, &block)
      assert_keys_with_block(convert_to_keys(keys), block)
      key = flatten_keys(keys)
      env_key = block.nil? ? key : block.()
      env_key = to_env_key(env_key)
      @envs[key.to_s.downcase] = env_key
    end
Bind a key to ENV variable

 @example
   set_from_env(:host)
   set_from_env(:foo, :bar) { 'HOST' }

 @param [Array[String]] keys
   the keys to bind to ENV variables

 @api public