Improve order implementation

[vasayxtx]

Hello! I use your bundle in my app, and I made a few changes:

• Use an order property instead an order() method (there are lot of seeds in my project and it's really shorter);
• Add the ability to configure bundle, that the order seeds are grown coincide with the order of files (e.g. php artisan plant::seed users,posts,user_post_links).

I find it convenient for me, maybe it will useful for somebody yet.

P.S.: Sorry for my English - it's not my native language.

[simshaun]

Thanks for the PR. I'm really busy at the moment but I'll take a look asap.

[simshaun]

So I finally got a chance to take a look at this.

I've got a couple observations:

1. I'd like to retain the ability to have an order() method for BC. Use the public property if it exists, otherwise, fall back to the order() method.
2. I don't want to require PHP 5.4

I've made a few comments on your commits. Take a look.