:bind-keymap - bind a key prefix to an autoloaded package keymap

John, 

Please consider this patch.  I've added the ability to bind a key sequence to a package's keymap.  Package loading is deferred until that key sequence has been pressed, at which time the package is transparently loaded so that the next key pressed is routed into the package's keymap.  

Here's an example of its use:
```
(use-package ahg :bind-keymap ("C-c h g" . ahg-global-map))
```