Add ignore_create_seed_file option

I don't want to update the seeds.rb in my application.

I add SeedMigration.ignore_create_seed_file.
If it's true, skip create_seed_file method.


I'm sorry my previous comment was both wrong and confusing.

We want to abort the method and `return` if `update_seeds_file` is `false` or if we're not in development environment.

So I think that the condition should be:

`if !SeedMigration.update_seeds_file || !Rails.env.development?`

Does that make sense?