Add identifiers prefix generation

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!

Start by reviewing issue 568 and the referenced webpack-obfuscator commit, then trace how this Gulp plugin passes options to javascript-obfuscator for each file. Done means generating an `aX` prefix per obfuscated file while preserving an explicitly supplied user prefix.

Written by the indexing model from the issue text.