Add i18nMark function for marking strings for extraction

[tricoder42]

Closes #60

[codecov]

Codecov Report

Merging #80 into master will decrease coverage by 0.01%.
The diff coverage is 93.1%.

@@            Coverage Diff             @@
##           master      #80      +/-   ##
==========================================
- Coverage    90.2%   90.18%   -0.02%     
==========================================
  Files          34       35       +1     
  Lines         786      805      +19     
==========================================
+ Hits          709      726      +17     
- Misses         77       79       +2

Impacted Files	Coverage Δ
packages/lingui-react/src/index.js	100% <100%> (ø)	⬆️
packages/lingui-i18n/src/index.js	100% <100%> (ø)
.../babel-plugin-lingui-extract-messages/src/index.js	97.59% <92.59%> (-2.41%)	⬇️

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[codecov]

Codecov Report

Merging #80 into master will decrease coverage by 0.01%.
The diff coverage is 93.1%.

@@            Coverage Diff             @@
##           master      #80      +/-   ##
==========================================
- Coverage    90.2%   90.18%   -0.02%     
==========================================
  Files          34       35       +1     
  Lines         786      805      +19     
==========================================
+ Hits          709      726      +17     
- Misses         77       79       +2

Impacted Files	Coverage Δ
packages/lingui-react/src/index.js	100% <100%> (ø)	⬆️
packages/lingui-i18n/src/index.js	100% <100%> (ø)
.../babel-plugin-lingui-extract-messages/src/index.js	97.59% <92.59%> (-2.41%)	⬇️

---

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 69b5d61...6c1b683. Read the comment docs.