Some handling reworks

This includes some minor stuff like:

* only output status info if both the version and source are not empty
* have rb print the status if no args are given

And some less-minor things like:

* move default .ruby-version file to `"$RB_HOME_DIR"`
* reset the ruby version with `rb @`

But the main overall goal is this:

```
Allows command line args to take precedence over parent
.ruby-version files yet have parent .ruby-version files override
command-line args when auto activating.
```

What this means is that if I specify a ruby version using an `@version` command line arg, it will remain in effect until I go to a dir with a parent .ruby-version file or I execute `rb @`.