Add method for raw parsing to providers

[zyuhel]

IMO public method should be added for Provider Class for parsing data.
Now there is extract($url) method, it calls _extract($url) submethod. This architecture make it's not possible for chaining processing. If all providers will have parse($html) method in addition to extract ($url)(which will call parse method itself after fetching data). it will be possible to chain providers. Fetch html one time, and then pass the returned data through more then one provider.