Add parameter `$defaultCategory` to `Translator` constructor

[vjik]

Q	A
Is bugfix?	❌
New feature?	✔️
Breaks BC?	✔️
Fixed issues	#77 (partly)

[codecov[bot]]

Codecov Report

Base: 94.75% // Head: 94.75% // No change to project coverage :thumbsup:

Coverage data is based on head (a3e2296) compared to base (9264cdb). Patch has no changes to coverable lines.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #82 +/- ## ========================================= Coverage 94.75% 94.75% Complexity 104 104 ========================================= Files 11 11 Lines 267 267 ========================================= Hits 253 253 Misses 14 14 ``` | [Impacted Files](https://codecov.io/gh/yiisoft/translator/pull/82?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yiisoft) | Coverage Δ | | |---|---|---| | [src/Translator.php](https://codecov.io/gh/yiisoft/translator/pull/82/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yiisoft#diff-c3JjL1RyYW5zbGF0b3IucGhw) | `92.72% <ø> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yiisoft). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yiisoft)

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