Add support for .po files

[tricoder42]

Closes #179

[codecov]

Codecov Report

Merging #256 into master will decrease coverage by 0.05%.
The diff coverage is 79.36%.

@@            Coverage Diff             @@
##           master     #256      +/-   ##
==========================================
- Coverage   91.92%   91.86%   -0.06%     
==========================================
  Files          39       41       +2     
  Lines        1003     1045      +42     
==========================================
+ Hits          922      960      +38     
- Misses         81       85       +4

Impacted Files	Coverage Δ
packages/cli/src/api/stats.js	0% <ø> (ø)	⬆️
packages/cli/src/api/compile.js	100% <ø> (ø)	⬆️
packages/cli/src/api/index.js	100% <100%> (ø)	⬆️
packages/cli/src/api/extract.js	100% <100%> (ø)	⬆️
packages/cli/src/mocks.js	100% <100%> (ø)	⬆️
packages/cli/src/api/formats/po.js	100% <100%> (ø)
packages/loader/src/index.js	100% <100%> (ø)	⬆️
packages/conf/src/index.js	96.42% <100%> (ø)	⬆️
packages/cli/src/lingui-extract.js	14.28% <12.5%> (-0.26%)	⬇️
packages/cli/src/api/formats/minimal.js	30% <30%> (+30%)	⬆️
... and 5 more

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