add use date number option to generate migration

* default
    * ``$ oil g migration foo x:string``
    * generate ``fuel/app/migrations/001_create_foo.php``
* when enable this option
    * ``$ oil g migration foo x:string``
    * generate ``fuel/app/migrations/1305111324_create_foo.php``
    * like a ruby on rails

Format is ``ymdHi``, isn't ``YmdHis``, because Migrate use 130511324 as a integer, on 32-bit system can't over ``2147483648``.

Signed-off-by: takyam <takayuki.yamaguch.m@gmail.com>