Add Yt::Search.

Thank you very much for putting this gem together! I'm glad to see a gem that uses the V3 API!

I looked through the docs and code, and to the best of my knowledge, there's no way of doing a keyword search. The plumbing for using `GET /search` is all there, but it seems to only be exposed in the context of retrieving the videos in a channel or account.

This PR adds a Search model that can be used to run a keyword query. It's very bare-bones, because I wanted to get your opinion before I sink more time into it. At the same time, it is covered by tests, and I'm using it successfully in an application.

What do you think?