add managing audience api

[justdomepaul]

add managing audience api to SDK.

Unit test is not finished.

Ref issue: Support Audience apis

[codecov-commenter]

Codecov Report

Merging #302 (7b873f2) into master (c7bd560) will increase coverage by 0.76%.
The diff coverage is 82.92%.

❗ Current head 7b873f2 differs from pull request most recent head fbab5a7. Consider uploading reports for the commit fbab5a7 to get more accurate results

@@            Coverage Diff             @@
##           master     #302      +/-   ##
==========================================
+ Coverage   78.53%   79.29%   +0.76%     
==========================================
  Files          37       38       +1     
  Lines        2874     3366     +492     
==========================================
+ Hits         2257     2669     +412     
- Misses        486      530      +44     
- Partials      131      167      +36     

Impacted Files	Coverage Δ
linebot/client.go	69.46% <60.97%> (-3.87%)	⬇️
linebot/response.go	68.96% <66.66%> (-0.68%)	⬇️
linebot/audience.go	88.39% <88.39%> (ø)
linebot/error.go	50.00% <0.00%> (+50.00%)	⬆️

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[kkdai]

Hi @justdomepaul
Would you please help to add some testing cases for audience.go ?

[justdomepaul]

@kkdai all right. Maybe wait me 3~5 days to add some test to coverage audience.go

[justdomepaul]

@kkdai I finish audience.go test case. Can you spend some time to review it?

[kkdai]

Thank you, LGTM