MSON AST

C++ implementation. For @zdne to review.

@fosrias and @honzajavorek can take a look if they want.
Absolutely. Please follow the pattern in `MarkdownNode` – the `members` should be private (https://github.com/apiaryio/markdown-parser/blob/master/src/MarkdownNode.h#L96) and you expose references via respective methods (https://github.com/apiaryio/markdown-parser/blob/master/src/MarkdownNode.h#L73)