Don’t override browser keyboard shortcuts on OS X

Nobody has claimed this yet.

In Firefox 28 on OS X, the [demo page](http://jaredly.github.io/treed/) swallows most keyboard shortcuts I try to use to control the browser. Any shortcut using a key that the demo uses is overridden, and interpreted as a command to the demo instead of to the browser, as if I had not held down Command. The most annoying overrides are ⌘L to focus the location bar and ⇧⌘{ and ⇧⌘} to switch tabs.

Where the code sets up keyboard shortcuts (probably [keys.js](https://github.com/jaredly/treed/blob/master/lib/keys.js) or [`attachListeners` in view.js](https://github.com/jaredly/treed/blob/ce7f68441b015af09b6f73a646ba85d964ae91cb/lib/view.js#L347-L357)), the state of the Command key should be checked when a keypress is detected. If Command is being held and the shortcut doesn’t require it, then the handler should not trap the keypress event.

Start in lib/keys.js and the attachListeners code in lib/view.js around lines 347-357, where keyboard shortcuts are detected. Check how the Command key is handled for shortcuts that do not require it. Done means browser shortcuts such as ⌘L and ⇧⌘{ or ⇧⌘} are no longer swallowed by the demo.

Written by the indexing model from the issue text.