Use `gulp-sourcemaps` for source maps

[DRSDavidSoft]

Description

This PR addresses a couple of issues with source maps:

• uses correct file names (fixes #13)
• is chainable with other plugins such as UglifyJS and Babel
• prevents plugins such as gulp-header from prepending to the source maps as well as the code

Because of https://github.com/javascript-obfuscator/javascript-obfuscator/issues/276, we can now use gulp-sourcemaps instead of throwing the source map to the stream! 😄

Backwards compatibility

As before, if gulp-sourcemaps is not used and obfuscator's sourceMap option is set to true, a .map file will be thrown to Gulp stream.

gulp.src('file.js')
    .pipe(javascriptObfuscator({
        compact: true,
        sourceMap: true
    }))
    .pipe(gulp.dest('dist'));

NOTE: The old method is not recommended for new projects, for the following reasons:

1. Any gulp plugin that is chained after gulp-javascript-obfuscator (e.g. gulp-header to prepend a banner to the output file) will be applied on both the obfuscated .js file and its .map file.

2. You will not be able to chain any other plugins to the task without loosing its own sourcemap file.

[Wain-PC]

Thank you, looks good to me!