Allow users to disable database_cleaner hooks

Per the discussion in #215, this PR provides an option controlling the automatic invocation of `DatabaseCleaner.start` before every scenario, and `DatabaseCleaner.clean` afterwards (it defaults to true, naturally):

```ruby
# features/support/env.rb
# ...
Cucumber::Rails::Database.autorun_database_cleaner = false
```

I added the option as an accessor on `Cucumber::Rails::Database` following the example of `.javascript_strategy` - it seemed the most natural place for it to live, but I'm happy to take alternative suggestions. :)

Cheers,
Simon