Support software keymaps

Big thanks to @NullVoxPopuli for this PR. Now, ember-keyboard will respond to the key mapping, rather than the key itself. For instance, with a Dvorak key mapping, pressing 'k' will give a code of 'KeyV' and a key of 'k'. Now, ember-keyboard will treat a Dvorak 'KeyV' as a 'k'.

This may introduce a regression if you intended to map to the key themselves, rather than to their code. If that's the case, please take out an issue and we'll figure out a way to support that behavior as well.