Support `magic-trace attach -p ... [cmd]` syntax

This would allow doing e.g.

```
$ magic-trace attach -p $(pidof foo) -trigger bar sleep 5
```

like `perf`.

Start at the command-line entry point for `magic-trace attach` and inspect how its options are parsed. Compare the requested `-p ... -trigger ... [cmd]` example with the existing attach behavior; done means an optional command after the attach options is accepted and executed while tracing the selected process.

Written by the indexing model from the issue text.