Add filter to the entries Export status

[pedro-mendonca]

Description

This allows to set a custom set of statuses, like 'current_or_untranslated'.

Fix #259

How has this been tested?

1. Set a different value on the status filter.
2. Check out the language packs as desired.

Types of changes
New feature (non-breaking change which adds functionality)

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code has proper inline documentation.

[swissspidy]

Once #270 is merged, this one should be good to go.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.78%. Comparing base (8a4a120) to head (1f646e1).

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #260      +/-   ##
============================================
+ Coverage     72.45%   79.78%   +7.32%     
  Complexity      350      350              
============================================
  Files            29       24       -5     
  Lines          1267     1014     -253     
============================================
- Hits            918      809     -109     
+ Misses          349      205     -144     

Flag	Coverage Δ
feature	?
php	79.78% <100.00%> (+0.01%)	⬆️

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