Add the async mode

[elszczepano]

The MVP version is synchronous. The reason for this is the predictability of prefix/postfix functions. In the async mode, it is not possible to create files in the following pattern: IMG-1.jpg, IMG-2.jpg, and so on.

However, it should be possible to enforce the async mode in cases when the feature described above is not needed.

The solution is the async flag that will parallelize renaming files.