support multiple type to hero

[ryota-sakamoto]

hero can set box or image type.
https://developers.line.biz/ja/reference/messaging-api/#bubble

[codecov-io]

Codecov Report

Merging #256 (5656604) into master (bb85a85) will increase coverage by 0.06%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master     #256      +/-   ##
==========================================
+ Coverage   77.44%   77.50%   +0.06%     
==========================================
  Files          34       34              
  Lines        2181     2196      +15     
==========================================
+ Hits         1689     1702      +13     
  Misses        383      383              
- Partials      109      111       +2     

Impacted Files	Coverage Δ
linebot/flex.go	58.09% <75.00%> (+1.17%)	⬆️
linebot/flex_unmarshal.go	70.43% <0.00%> (+1.73%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update bb85a85...5656604. Read the comment docs.

[kkdai]

LGTM