Add offset parameter

[nick15marketing]

Our tables have a lot of data and would be good to be able to split up seed files into more manageable sizes. If offset was added it could be used in conjunction with max and suffix to easily split into smaller files eg:

php artisan iseed my_table --max=500 --offset=0 --classnamesuffix=A
php artisan iseed my_table --max=500 --offset=500 --classnamesuffix=B

[nick15marketing]

Just noticed there is already a PR for this:
https://github.com/orangehill/iseed/pull/143/files