use hyphens instead of underscores

I think USAGE output should be changed. See:

```
$ alr action --help
SUMMARY
   List or manually trigger action hooks

USAGE
   alr action [options] [post_fetch|pre_build|post_build|test]

$ alr action post_fetch
error: TOML keys should use hyphens instead of underscores, but found key: post_fetch
```
This command runs fine, however:

    $ alr action post-fetch
