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?
is this planning to change later on?

Maybe:
```
ipfs pin ls [-r, --recursive] [-i, --indirect]
# or
ipfs pin ls [-t, --type [ recursive | indirect | direct | all ] ]
```