Tracker Announce Subcommand Follow-up

There are some things I think would be nice to have for `imld torrent announce`, but I think they belong in follow-up diffs, to keep the initial diff size. I'm putting them in the same issue so I can keep track of them, but they could easily be broken out into separate issues.

- The warning for non-UDP tracker URLs should be formatted `warning: …` with `warning` in yellow.
- It should be possible to pass just an infohash and tracker URL(s) to the CLI (and not a torrent)
- There should be a flag to produce JSON output:

```
[
{
"URL": URL,
"PEERS": [
{"IP": IP, "PORT": PORT},
…
]
},
…
]
```