Improve choice to allow jumping to queries/forms

[GochoMugo]

We are having so much code looking like this:

{
    "name": "test",
    "question": {
        "text": "test query",
        "choices": [{
            id: 1,
            text: "choice 1",
        }, {
            id: 2,
            text: "choice 2",
        }],
    },
    pre(answer, done) {
        switch (answer) {
        case 1: return this.do("form1", done);
        case 2: return this.do("form2", done);
        }
    },
}

which could be shortened to:

{
    "name": "test",
    "question": {
        "text": "test query",
        "choices": [{
            id: 1,
            text: "choice 1",
            do: "form1",
        }, {
            id: 2,
            text: "choice 2",
            do: "form2",
        }],
    },
}

Adding do (and goto) props to choices should allow jumping to
a form (and query) when the choice is matched.