From discussion with @carlos-zamora and @DHowett-MSFT, and possibly in #968

Keybindings need to support arbitrary args, so that the handlers can parse their own arg values.

{
    keys: ["ctrl+shift+c"],
    action: copy,
    args:{
        type: "mungeData"
    }
},
{
    keys: ["ctrl+shift+pgup"],
    action: scroll,
    args:{
        amount: -1
    }
}

This is a feature that needs to absolutely be spec'd before implemented.