Implement po_create()/po_update() for creating/updating translations

[hadley]

I don't love the name tr_add(), and it's making me doubt that the developer/translator distinction is that useful, because this function might be run by the developer or an R-packages savvy translator.

I tweaked the .Rproj to make my usual roxygen2 workflow a little smoother; let me know if you want to revert.

I'm little surprised that potools doesn't already use msginit, so maybe I should instead copy the approach used in translate_package()?

[hadley]

Changed it to add or update, depending on whether or not the .po file already exists. That makes tr_add() and even worse name.

[hadley]

I think this is better as po_create() and po_update(). I think po_update() should update all languages if none supplied; but that needs code from #234, so I'll wait until you've have a chance to review that.

[MichaelChirico]

I'm little surprised that potools doesn't already use msginit

There are a couple of gettext tools I would do well to integrate better with / provide good wrappers for...

[codecov-commenter]

Codecov Report

Merging #235 (a088663) into master (d68b90b) will decrease coverage by 1.51%. The diff coverage is 22.22%.

@@            Coverage Diff             @@
##           master     #235      +/-   ##
==========================================
- Coverage   98.80%   97.29%   -1.52%     
==========================================
  Files          17       17              
  Lines        1341     1367      +26     
==========================================
+ Hits         1325     1330       +5     
- Misses         16       37      +21     

Impacted Files	Coverage Δ
R/msgmerge.R	58.20% <22.22%> (-24.72%)	:arrow_down:

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[hadley]

Ok, reimplemented basically from scratch; I'll add a bunch of tests shortly.

[MichaelChirico]

Looking great, thanks!