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
```
Lets make this required, and i'm happy to merge.

I'm 99% certain we'll want this in all cases, and an optional version will add more confusion than benefit. Probably best to do this sooner, we can update the current Smartdown examples that will be broken.