Added only_links_like Feature

Hi!

Thanks a lot for anemone guys!

I missed a feature that I have implemented, basically the reverse of `skip_links_like`, which I have called `only_links_like` to help the crawler to follow only links which matches specified patterns.

Anyway, I wish you great succes with your project!

Best,
Julian
```ruby
@only_link_patterns.empty? 
 ? @skip_link_patterns.any? { |pattern| link.path =~ pattern }
 : !@only_link_patterns.any? { |pattern| link.path =~ pattern }
```
@hartator would u care to re-submit in the [Medusa fork](https://github.com/brutuscat/medusa/)?