Allow objects to be passed as a parameter on the worker-pool method

[pepjo]

I was using your module but I found that I would just love to pass the name of the file gulp needed to minify as a parameter.

I made some modifications in order to do so. Now you can pass a second parameter to the 'makeItFaster' function with some parameters. It's important to notice that I use JSON.stringify, so no functions can be passed.

The parameter is then passed as 'this' to the function.

Code example:

var makeItFaster = require('spawn-task-experiment').workerPool();

gulp.task('whatever', makeItFaster(function() {
    var gulp = require('gulp');
    var sourcemaps = require('gulp-sourcemaps');
    var concat = require('gulp-concat');

    //We get the name
    var name = this.name;

    return gulp.src(['app/' + name + '.js', 'app/**/*.js'])  //We use name
        .pipe(sourcemaps.init())
        .pipe(concat('all.js'))
        .pipe(sourcemaps.write('.'))
        .pipe(gulp.dest('.build/scripts'));
}), 
{
  name: 'hello'
});

PS: This is my first pull request. I hope I did it right.