Simplify MultipleChoice model

it had incorporated some logic which has to do with parsing raw input
data. This is a concern of the frontend app. It can actually be a dumb
struct and as such doesn't really need its own spec.
Add MarkdownHeading model
Add MarkdownParagraph model
Better rule name
Refactor rules parser to include next_node_rules capture

Fits with pattern used by other element parsers
Remove unnecessary requires
Merge pull request #6 from alphagov/markdown-models

Markdown models