Add hardware keyboard shortcuts

[frosty]

Let's add keyboard shortcuts for the various editor features for devices with a hardware keyboard, like the iPad Pro with a Smart Keyboard.

We can do this by implementing UIResponders' keyCommands method in the editor view controller and returning a list of UIKeyCommands. Any commands that have a discoverabilityTitle will show up in the iOS overlay that appears when you hold down a modifier key on an external keyboard:

At the very least, we could add shortcuts that correspond to each of the items in the editor toolbar:

• Insert image (Cmd+Shift+I?)
• Bold and Italic we get for free (Cmd+B, Cmd+I), but I think we could add explicit keyboard shortcuts so that they show in the overlay
• Strikethrough (Cmd+Shift+S? Cmd+~?)
• Quote (Cmd+>?)
• Bullet list (OneNote uses Cmd+., iA Writer uses Cmd+L)
• Ordered list (OneNote uses Cmd+/, iA Writer uses Cmd+Shift+L)
• Insert link (Cmd+K seems to be a fairly standard shortcut for this – it's what TextEdit uses)
• Toggle HTML mode (Cmd+<? Cmd+H?)

I can't find examples of pre-existing shortcuts for a number of these items in other apps (where I've used a ? above), so we'll just have to choose pick an appropriate shortcut in those cases.