Added support for searches filtered by available market

I need to filter searches by the available market (as seen [here](https://developer.spotify.com/web-api/search-item/)) so that the results are playable for the user based on their location. This is my first Ruby PR so I would appreciate any feedback!

I was wondering if I should update all the search test cassettes to make sure they all still work properly with this new change? Also, my editor stripped trailing whitespace, I can put it back if the trailing whitespace was supposed to be there.
It's hard to test this because no "available_markets" are returned