`torrent create` command

This command uses [create-torrent](https://www.npmjs.org/package/create-torrent) to create torrent files. If you want to create a torrent file for a directory `xyz/`, you can just do:

```
torrent create xyz/ -o xyz.torrent
```

If `-o` isn't specified, the torrent file is written to stdout. I also added a usage.txt that is displayed when you `torrent --help`.