Support "Send broadcast api"

[kkdai]

Sends push messages to multiple users at any time.

Refer https://developers.line.biz/en/reference/messaging-api/#send-broadcast-message

[codecov-io]

Codecov Report

Merging #151 into master will increase coverage by 0.24%.
The diff coverage is 91.3%.

@@            Coverage Diff             @@
##           master     #151      +/-   ##
==========================================
+ Coverage   72.39%   72.63%   +0.24%     
==========================================
  Files          23       23              
  Lines        1782     1805      +23     
==========================================
+ Hits         1290     1311      +21     
- Misses        414      415       +1     
- Partials       78       79       +1

Impacted Files	Coverage Δ
linebot/client.go	74.32% <ø> (ø)	⬆️
linebot/send_message.go	92.07% <91.3%> (-0.23%)	⬇️

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[kkdai]

fixed #156

[kkdai]

Fixed for #158

[kkdai]

Hi @k2wanko
Could you help to check this PR? Thank you.

[notsu]

Hi @sugyan,

Could you please review this PR? This PR might be useful for every developer to use broadcast API 😁

[k2wanko]

@kkdai Thank you.