Implement handy interface for configuring filtered tables

handy interface for configuring `:except` and `:only` discussed #18 and asakusa.rb meetup.

```ruby
# Usage
DatabaseRewinder.filter_options = except: ["foos"]
DatabaseRewinder.clean_all
``` 

I named `filter_options=`, but I'll leave other candidates :memo:
* filter_settings
* filtered_tables
* filtered_table_options
* filter_table_options
* table_options
* tables_to_filter
* filtering_options
* filtering