Cross-platform environment variable signature, @

On Unix, environment variables are referenced via the `$` character. On windows, its `%`. Which means we can't make use of either one in a cross-platform compatible library.

``` bash
# Posix
$ echo $ROOTDIR
'/studio'

# Windows
$ echo %ROOTDIR%
'c:\studio'
```

How about `@`?

``` bash
$ dash @ROOTDIR
'c:\studio'
```
