"Open With" support

Several users have requested the ability to open files with a non-default editor (i.e. not the one picked by NSWorkspace, which is derived from Finder associations).

So far we've resisted this because Apple say you're not supposed to do it, and indeed the Sandbox expressly prevents it. However, since we're now no longer going to try to satisfy the Sandbox requirements and are not committing to continuing App Store support, we could potentially implement this.

Of course if we reviewed our App Store / Sandboxing position later, this would be one more thing we'd have to turn off.