Add identifiers prefix generation

[sanex3339]

Hi.
javascript-obfuscator/javascript-obfuscator#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:
javascript-obfuscator/webpack-obfuscator@2b227b2#diff-ed009b6b86b017532ef0489c77de5100

Thank you!