FEAT: define ~ as a shortcut for system/options/home

So it is like in *nix shells and one can use it like:
```
>> cd ~
== %/X/Rebol/

>> cd ~/..
== %/X/

```

Author: Oldes

src/mezz/mezz-shell.r

@@ -15,6 +15,7 @@ ls:		:list-dir
 pwd:	:what-dir
 rm:		:delete
 mkdir:	:make-dir
+~: system/options/home
 
 cd: func [
 	"Change directory (shell shortcut function)."