Adds date formatting.

Essentially, this is the same function as `grunt.template.date` or `grunt.template.today`. Don't think we need the extra `today` method as that case is handled pretty well by the library anyway. Sample usage could be:

```
gulp.task('default', function() {
    gulp.run('lint');
    gulp.watch('*.js', function(event) {
        gutil.log(gutil.colors.cyan(event.path) + ' ' + event.type + '. (' + gutil.date('HH:MM:ss') + ')');
        gulp.run('lint');
    });
});