add field label to imagemap actions

[TonPC64]

https://developers.line.biz/en/reference/messaging-api/#imagemap-action-objects

[codecov-io]

Codecov Report

Merging #176 into master will increase coverage by 0.08%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #176      +/-   ##
==========================================
+ Coverage   73.41%   73.49%   +0.08%     
==========================================
  Files          24       24              
  Lines        1903     1909       +6     
==========================================
+ Hits         1397     1403       +6     
  Misses        423      423              
  Partials       83       83

Impacted Files	Coverage Δ
linebot/imagemap.go	94.44% <100%> (+1.11%)	⬆️

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[oklahomer]

The document says this field is optional. Can we include "omitempty" in this tag just like other Actions such as PostBackAction, URIAction, etc?

[oklahomer]

Same as https://github.com/line/line-bot-sdk-go/pull/176/files#r344946868

[oklahomer]

I appreciate your effort to add go.mod. However, I believe a single pull request should focus on a single feature/issue. Especially when the new requirements include google.golang.org/appengine, which actually seems to be optional dependency to run examples/echo_bot_handler with build tag of -tags appengine.
So could you remove this change from this p-r? Other than that, those changes for labels look good.

[TonPC64]

I appreciate your effort to add go.mod. However, I believe a single pull request should focus on a single feature/issue. Especially when the new requirements include google.golang.org/appengine, which actually seems to be optional dependency to run examples/echo_bot_handler with build tag of -tags appengine.
So could you remove this change from this p-r? Other than that, those changes for labels look good.

ok removed

[oklahomer]

Thank you for the additional changes 👍