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.


```if SeedMigration.update_seeds_file && !Rails.env.development?```

I think skip update seeds if SeedMigration.update_seeds_file is false.
Correctry ```if !SeedMigration.update_seeds_file && !Rais.env.development?``` ?

I think better

```ruby
def self.create_seed_file
  return if !Rails.env.development?
  return if !SeedMigration.update_seeds_file
```

or

```ruby
def self.create_seed_file
  unless Rails.env.development?
  unless SeedMigration.update_seeds_file
```

what do you think?