Feature/clarify misc scenarios

@zdne @pksunkara @smizell @honzajavorek Please review and merge.

This PR clarifies a number of scenarios that we ambiguous or missing when updating examples in https://github.com/apiaryio/mson/pull/6. 

- Adds quick links
- Cleans up structure of spec to make more sense in context of hierarchical list.
- Clarifies escaping keywords
- Adds wildcard type name
- adds Default functionality and aligns with Sample functionality with 3 flavors each

DO NOT MERGE until https://github.com/apiaryio/mson/pull/6 is merged.

The default is mutually exclusive with required and also mutually exclusive with sample. Do we need to capture this somewhere?