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!

先檢視 issue 568 和所引用的 webpack-obfuscator commit，接著追蹤這個 Gulp plugin 如何針對每個檔案將選項傳遞給 javascript-obfuscator。完成條件是為每個經過混淆的檔案產生一個 `aX` 前綴，同時保留使用者明確提供的前綴。

由索引模型根據 Issue 內容生成。