Adding conditional logic to forms

Branching can be done when defining your parsers. Should be a 1 liner. Perhaps we'll use the term "requiredIf" or "inScopeIf". Approach is collect a lot of data on forms in the wild and find the smallest change we can make to support most/all of them.

```
nameParser
extends abstractIdParser
description What is your name?

specialtyCell
enum Frontend Backend

specialtyParser
extends abstractEnumMeasureParser
cells commandWord specialtyCell
description What is your specialty?

frontendParser
extends abstractUrlMeasureParser
description What is your favorite JS frontend?
requiredIf specialty = Frontend
```