Support optional question tag for multiple choice questions

The syntax is:

```
[choice: question_name]
* opt1: Opt label 1
* opt2: Opt label 2
```

this will create a multiple choice question with the given name. The
label is optional and if unspecified the question assumes the name of
the node. Written as:

```
* opt1: Opt label 1
* opt2: Opt label 2
```
yes, and changing the specs

I wanted to keep it backward compatible for now, but if you think it wouldn't be disruptive to make it mandatory, I'd be happy to do so.