Add absolute option

I added `:absolute_path` option to `FeedSearcher.search`.
It makes results of `FeedSearcher.search` absolute URLs.

```ruby
FeedSearcher.search('http://hitode909.hatenablog.com')
  # => ["/feed"]
FeedSearcher.search('http://hitode909.hatenablog.com', :absolute_path => true)
  # => ["http://hitode909.hatenablog.com/feed"]
```