Add identifiers prefix generation

[sanex3339]

Hi. https://github.com/javascript-obfuscator/javascript-obfuscator/issues/568

It would be nice if you add a unique identifier names prefix generation for each obfuscated file. The prefix should be as aX, where X - number or counter of the current obfuscated file (0, 1, 2, ...)

The prefix can be overridden by the user prefix.

I just implemented this logic for webpack-obfuscator, you can see example: https://github.com/javascript-obfuscator/webpack-obfuscator/commit/2b227b27bc1e10c687ed656d25a9efb6d16f1a0f#diff-ed009b6b86b017532ef0489c77de5100

Thank you!