Merge pull request #12 from alphagov/conditional

Support for conditionals in outcomes
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
```
Make multiple choice question tag required

We felt that it would be more confusing to let it be optional
Update student finance forms example

[choice: ] tag is now required
Update check-uk-visa example

choice tag now required
Merge pull request #11 from alphagov/optional-choice-question-tag

Support optional question tag for multiple choice questions
Add instructions about the use of conditionals
Improve error message on undefined value
Add interpolator
Rename NodePresenter to ConditionalResolver
Refactor node presenter to invoke conditional resolver
Integrate variable interpolation to Engine#evaluate_node
Add interpolation instructions to readme