Language mapping is not supported by the CLI

[JanCizmar]

The Apple's String Catalog (.xcstrings) contain all the languages in a single file, so we need a way to map the languages from such file to the languages in Tolgee.

For xliff and xcstrings, the current docs does make much sense, because there is a single language property, while we need to map more languages from such files.

I am currently not sure, which configuration to provide when I want to push only base language or when I want to push only specific languages from such file.

The docs should reflect this kind of special formats.

There is my draft of PR for the xcstrings support containing an example of such file: #153