Subcommand to edit existing torrent files

- [ ] `imdl torrent edit`
- [ ] allow editing source files
- [ ] set `info.entropy`.
- [ ] Check out https://github.com/rndusr/torf-cli for inspiration
- [ ] --dry-run to show what would be done
- [ ] --clean to remove nonstandard metadata
- [ ] --anonymize remove anything that might be identifiable
- [ ] --public and --private to change `private` flag

A great first stab at this would be to add the subcommand, and add the ability to set the source of a torrent, with something like:

```
$ imdl torrent edit --input foo.torrent --set source foo
```

Setting the source of a torrent is likely to be one of the main uses of editing torrents, so it's a good place to start.

I'd like this to use a relatively small number of generic flags:

- `--set NAME VALUE` set the given named field to `VALUE`. `NAME` should be the name of a field, not a path, so `info.source` should be set with `--set source FOO`.
- `--remove NAME` remove the given field, if set
- `--update NAME` add or update a field that has a default, like `created by`, or `creation time`

Ideally, the same flags should also be accepted by `imdl torrent create` and `imdl torrent link`