Implement os-specific actions?

Should we allow for actions to indexed by os? (and arch? other things?)

For example:

```yaml
actions:
  clean@posix: "rm -f classes"
  clean@win: "rd /s /q classes"
```

To take note of:
- more specific wins over less
- need to deal with cases where an action exists, but not for the os it's being asked to run on
