Support for other structures

[sewcio543]

Abstraction over implementation!
At the moment, soupsavvy is search engine for specific bs4 implementation.
When interface is extracted and used in the library, different implementation can be used as input to selectors.
With adapters following the same interface, the same operations will be available for lxml or selenium elements.
This change will make library more versatile.
As much backward compatibility as possible is advised, but major release can be considered for that, as first step into new direction.