Implement AST media types v2.0, deprecate resource and action section's headers

### Breaking

This PR includes **backward incompatible changes to serialized AST**. Please refer to [API Blueprint AST Serialization Media Types  v2.0 Release](https://github.com/apiaryio/api-blueprint-ast/releases/tag/v2.0) for details on braking changes in this version of AST. 

In addition to the changes in serialization this PR deprecates resource and action section headers subsection.