Allow additional configuration options and enhanced control when using `reap`

#### Additional Options (`:limit` and `:ignored_attrs`)
```
# rake
rake db:seed:reap ENV=integration MODELS=User, Post LIMIT=10 IGNORED_ATTRS=created_at, updated_at

# console
Sprig::Harvest.reap(
  :env           => 'integration',
  :models        => [User, Post],
  :limit         => 10,
  :ignored_attrs => [:created_at, :updated_at]
)
```
#### File-Based Config (allows for model-by-model configuration)
```
# db/seeds/reap_config.rb

Sprig::Harvest.configure do |config|
  config.env           = 'dreamland'
  config.limit         = 30
  config.ignored_attrs = [:created_at, :updated_at]

  config.models = [
    {
      :class         => User,
      :collection    => User.active.where(:admin => false),
      :limit         => 10,
      :ignored_attrs => :password
    },

    {
      :class      => Post,
      :collection => Post.published,
      :limit      => 20
    },

    Comment
  ]
end
```
To use this reap config file, invoke `reap` via:
```
# rake
rake db:seed:reap FILE="./db/seeds/reap_config.rb"

# console
Sprig::Harvest.reap(:file => './db/seeds/reap_config.rb')
```
:point_up: It would be much clearer if

```ruby
def limit=(value)
  limit = Integer.parse(value)
  fail "Limit must be > 0" if limit <= 0
end
```