other sub-key in translation may cause a key to be marked as unused

[ClearlyClaire]

I'm running into an issue similar to #473, but with the unused check.

This occurs when a source item has an other, and a translation is partial and has only the other key:

en.yml:

---
en:
  preferences:
    other: Other
    posting_defaults: Posting defaults

ga.yml:

---
ga:
  preferences:
    other: Eile

This causes i18n-tasks unused to incorrectly flag the translation as unused:

Unused keys (58) | i18n-tasks v1.0.12
+--------+-------------+--------------------+
| Locale | Key         | Value              |
+--------+-------------+--------------------+
|   ga   | preferences | {"other"=>"Eile"}  |
+--------+-------------+--------------------+

Adding 'preferences.other' to ignore_unused works, but it would be preferable if i18n-tasks could detect this is not a plural key based on use or reference locale, or failing that, have an option to mark known keys as non-plural, but not preventing the unused key check if they truly get unused at some point.