Add shortcut for whitelisting all modifier keys

For an action subscribing to an event regardless of modifier keys, the template

```handlebars
<a href="#" {{action "edit" allowedKeys="alt shift meta ctrl"}}>click me</a>
```

becomes

```handlebars
<a href="#" {{action "edit" allowedKeys="any"}}>click me</a>
```

This lends itself to a cleaner and more future-proof template. Thoughts?