flo status command

It'd be nice to turn `flo run --dry-run` into a `flo status` command. This could make it possible to also build a web interface that makes it easy to view the task tree to make it easy for people to understand dependencies in a useful way. This would be a significant advantage over several other options out there. Here's a prototype UI:

```bash
flo status                # prints out current `flo run --dry-run` output
flo status --serve [PORT] # starts a webserver to show task hierarchy
```

This has the added benefit of making it harder for people to inadvertantly execute `flo run` when they intended to add the `--dry-run` flag. Perhaps `flo run --dry-run` can remain a synonym for `flo status`, but I think its better to have a separate subcommand for that.