Support for LibreTranslate

[tacman]

I've been using LibreTranslate recently, and really quite happy.

For some projects, I use it during development, because it's fast, local, and free. Then I switch to DeepL (#19) if I'm okay paying for it.

If you can provide some guidance for integrating a new translator, I can possibly add both, since I'm using them now.

[bocharsky-bw]

I've heard good feedback about DeepL. If you want to help with adding their support in this library - please, see already written translator implementations. I suppose you want to do something similar. As you will see, you need to implement a special interface and so implete specific methods in your translator classes