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!

首先检查 issue 568 和所引用的 webpack-obfuscator 提交，然后追踪此 Gulp 插件如何针对每个文件将选项传递给 javascript-obfuscator。完成标准是为每个经过混淆的文件生成一个 `aX` 前缀，同时保留用户明确提供的前缀。

由索引模型根据 Issue 内容生成。