Added 'pin ls' Command

This PR adds a `ipfs pin ls` command, where users can view a list of what is being pinned. It only shows direct pins (keys that have been added with `ipfs pin add`). Maybe we want it to show the indirect/recursive pins as well?
Ok, we can solve it by adding a `Recursive` modifier to `commands.Option`, to specify an option that tells the commands lib to allow recursive files (kind of like how we already have one to enable recursive files for a certain argument).