Improve order implementation

Hello! I use your bundle in my app, and I made a few changes:

+ Use an `order` property instead an `order()` method (there a 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.