Add flag to ignore POT-Creation-Date for updates

[joeportela]

Adding a flag to ignore changes to POT-Creation-Date when performing updates or checks.
I use a CI check that ensures my .po files are updated before merging, but there is always a diff on POT-Creation-Date.

Related to #725 - but solving (my) problem via update instead of extract.

[joeportela]

Ping @akx
Any interest in this PR?

[akx]

Thank you for the contribution! As mentioned inside, I think it could be more intuitive for the kwarg to be positive (and default to True).

[joeportela]

Thank you for the contribution! As mentioned inside, I think it could be more intuitive for the kwarg to be positive (and default to True).

My pleasure, thanks for taking a look! Just updated per your suggestion.

[codecov]

Codecov Report

Merging #999 (620fc1d) into master (1747d22) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #999   +/-   ##
=======================================
  Coverage   90.90%   90.91%           
=======================================
  Files          25       25           
  Lines        4356     4358    +2     
=======================================
+ Hits         3960     3962    +2     
  Misses        396      396           

Impacted Files	Coverage Δ
babel/messages/catalog.py	95.72% <100.00%> (+0.01%)	⬆️
babel/messages/frontend.py	87.40% <100.00%> (+0.02%)	⬆️

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[akx]

LGTM, sorry for the delay!