Add redelivery object for narrowcast

[hinoguma]

#230
https://developers.line.biz/en/reference/messaging-api/#narrowcast-recipient

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@b4c3a9c). Click here to learn what that means.
The diff coverage is 75.00%.

@@            Coverage Diff            @@
##             master     #242   +/-   ##
=========================================
  Coverage          ?   77.07%           
=========================================
  Files             ?       34           
  Lines             ?     2146           
  Branches          ?        0           
=========================================
  Hits              ?     1654           
  Misses            ?      384           
  Partials          ?      108           

Impacted Files	Coverage Δ
linebot/recipient.go	80.76% <75.00%> (ø)
linebot/emoji.go	100.00% <0.00%> (ø)
linebot/error.go	0.00% <0.00%> (ø)
linebot/get_summary.go	100.00% <0.00%> (ø)
linebot/get_profile.go	100.00% <0.00%> (ø)
linebot/get_content.go	100.00% <0.00%> (ø)
linebot/get_count.go	100.00% <0.00%> (ø)
linebot/client.go	68.00% <0.00%> (ø)
linebot/actions.go	73.33% <0.00%> (ø)
linebot/delivery.go	70.37% <0.00%> (ø)
... and 25 more

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[kkdai]

LGTM, thank you.