Add arguments to custom actions

[electrikmilk]

This refactors custom actions to allow for argument definitions that can then be used later to pass arguments to the defined action.

This makes the following syntax possible:

action add(number op1, number op2) {
    const add_expression = op1 + op2
    output("{add_expression}")
}

const result = add(2,2)
show("{result}")

I'm not crazy about this because it introduces syntax that does not translate directly to shortcuts but having this functionality is very important to users of the language and the prospect of a standard library of custom actions is very interesting to me.