Retain custom flags during merge

[johantell]

Background:
In our project, we've implemented an auto-translate feature that helps team members translate to languages they don't know (and we'll help adjust them through the review process or send them over to someone who helps us with the translation).

We've set up a mix task that back-fills all empty translations in a specific language with translations from an external language model and adds the auto-translated tag to the message.

We've noticed recently that all those tags get removed when doing a mix gettext.extract --merge, which causes some disruption for us as we use those tags to know which messages we should send to our external human translators for verification.

Problem
From my understanding (by looking through the Merger module), fuzzy is the only flag allowed to be retained over multiple merges (https://github.com/elixir-gettext/gettext/blob/main/lib/gettext/merger.ex#L230-L236), which is why we're experiencing our problem

Suggested solution
I understand that there is most likely a very sound reason to resetting the tags on each extraction so my suggestion is that we make the retained titles configurable through our mix-file:

def project do
  [
    #...
    gettext: [
      retain_flags: ["fuzzy", "auto-translated"]
    ]
    #...
  ]
end

Where we either use ["fuzzy"] as a default or merge the "internal" flags with the configured value

---

If this suggestion sounds good to you, I'd be happy to implement it and shoot a PR your way :)

[whatyouhide]

@johantell yes, the retain_flags option sounds great. Let's have fuzzy in there by default and keep it in there though, so that you don't have to configure it manually. In that case, we can call the option :custom_flags_to_keep, wdyt?

[johantell]

Sure thing, :custom_flags_to_keep sounds good 👍

[whatyouhide]

@johantell awesome. PR is definitely appreciated!! 💟