Folder feature

Hello again,

So I decided to add Folder feature into the pop gem. How its works ? If you try to add a key with `pop add` then its cheks if this is vaild url, if not then it will be folder. For folder opening i used osascript to open new terminal tab and open a folder. Also, I added rspec gem. Please check and tell your opinions, what need to change, add and more.

P.S. Don't merge this now.