Add support for custom "prefixFileName" function

[ghost]

For now, prefixes are generated using the hardcoded function which is not sufficient for some more complicated cases.

Let's assume the following input files:

• example1.en.json
• example2.en.json

With true passed to the prefixFileName option, the result will be {"example1.en": {...}, "example2.en": {...}}. But when input files are grouped by the language-based pattern, the .en postfix is redundant.

The proposed solution is backward compatible and does not introduce any breaking change. Only if a function is passed as the prefixFileName parameter, it will be called to generate a prefix.

[tettusud]

@pwachala Awesome, thank you much for taking time to make this code better!