Allow customize message for missing translations

[tricoder42]

Closes #186

[codecov]

Codecov Report

Merging #255 into master will increase coverage by 0.24%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #255      +/-   ##
==========================================
+ Coverage   91.68%   91.92%   +0.24%     
==========================================
  Files          39       39              
  Lines         998     1003       +5     
==========================================
+ Hits          915      922       +7     
+ Misses         83       81       -2

Impacted Files	Coverage Δ
packages/cli/src/api/formats/lingui.js	94% <100%> (-0.12%)	⬇️
packages/core/src/i18n.js	100% <100%> (ø)	⬆️
packages/react/src/I18nProvider.js	100% <100%> (ø)	⬆️
packages/react/src/Trans.js	93.75% <0%> (+12.5%)	⬆️

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