Remove Nokogiri dependency by using Oga as an alternative HTML parser

As mentioned in #105, sometimes installing Nokogiri could be the hardest part of setting up gems.

I've tried to remove Nokogiri dependency by using another html parser [Oga](https://github.com/YorickPeterse/oga) which has been introduced lately.
Although Oga is a bit still young, i thought it's worth to think about it.