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:

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

[swissspidy]

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

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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](https://app.codecov.io/gh/wearerequired/traduttore/pull/260/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=wearerequired) | Coverage Δ | | |---|---|---| | [feature](https://app.codecov.io/gh/wearerequired/traduttore/pull/260/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=wearerequired) | `?` | | | [php](https://app.codecov.io/gh/wearerequired/traduttore/pull/260/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=wearerequired) | `79.78% <100.00%> (+0.01%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=wearerequired#carryforward-flags-in-the-pull-request-comment) to find out more.

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