Allow letters to be used for game control; disabled during input.

On my system configuration, `F1`, `fn+F1`, `F2`, and `fn+F2` are all reserved for system functions. So, within the context of the game, it's not _possible_ to type `F1` and have that input passed to the browser.

This revision does two things:

* Allows multiple key bindings to be attached to any given action. For example, both `LEFT` and `j` will move the character left.
* Revises active bindings depending on input context. This removes the need for using preventDefault. Instead, appropriate bindings are removed during the text input context, then added back when switching to the movement context.

All default bindings have been retained to avoid mismatch with current game instructions.

These bindings have been added:

```
j: left
l: right
i: up
k: down
h: help
m: mute/unmute
```