recursively expands ${VARIABLES} within the value (w/o refactor)

My editor ate trailing spaces, but it should be clean enough.  Fixes #39.
Is this just here to allow for 

```
FOO=${BAR} // this will be "baz"
BAR=baz
```
I think it is reasonable for that variable not to resolve. Its not a big deal either way. @maxbeatty you have an opinion on this.