Update to 4.0 and automatically generate identifiersPrefixes

[softworkz]

Generation of identifiersPrefix values per file is required as requested here already #20

to avoid errors like these:

• Identifier has already been declared with mangled option javascript-obfuscator#568
• Version: 4.0.0. Error: Evaluation failed: ReferenceError: _0x5b9d0e is not defined with config stringArray: true javascript-obfuscator#1093
• Flutter main.dart.js compilation results in runtime exception - Uncaught TypeError: Instance of '_0x25ae98': type '_0x25ae98' is not a subtype of type 'JavaScriptObject' javascript-obfuscator#1150
• myfunction[_0x2d7ba2(...)] is not a function happened after obfuscator javascript-obfuscator#1172

[softworkz]

As I don't have time to wait for whether this might ever get merged or not, I have created

a fork: https://github.com/softworkz/grunt-contrib-obfuscator-4
and a new Grunt plugin: https://www.npmjs.com/package/grunt-contrib-obfuscator-4