Add URIAction.AltURI.Desktop

[sugyan]

Resolve #143.

• Added URIAction.AltURI as optional property.
• Added tests to flex_test.go (because altUri property is supported only in Flex Messages).
• Update kitchensink server
  • When the user says "flex json" command, the bot replies flex message with two buttons. When opened in the desktop version, the second link will open a different URI than the mobile version.

[codecov-io]

Codecov Report

Merging #147 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #147      +/-   ##
==========================================
+ Coverage   72.07%   72.11%   +0.03%     
==========================================
  Files          23       23              
  Lines        1755     1757       +2     
==========================================
+ Hits         1265     1267       +2     
  Misses        413      413              
  Partials       77       77

Impacted Files	Coverage Δ
linebot/actions.go	73.1% <100%> (+0.45%)	⬆️

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[suzuki-shunsuke]

LGTM