Allow environment variables

[thecaddy]

Per http://nodejs.org/api/child_process.html#child_process_child_process_spawn_command_args_options should allow passing environment variables.

I'd make a pull request but I'm lazy. Here is the only change line 38 of index.js:

// spawn program
        var program = cp.spawn(options.cmd, options.args, options.opts);

Will allow me to do this:

gulp.src("./src/images/*.{jpg,png,gif}", { buffer: false })
    .pipe(spawn({
        cmd: "convert",
        args: [
            "-",
            "-resize",
            "50%",
            "-"
        ],
       //ENV
       opts: { env: {"NODE_ENV":  "booya"} },
        // optional
        filename: function(base, ext) {
            return base + "-half" + ext;
        }
    }))
    .pipe(gulp.dest("./dist/images/"));