Implement plugginable architecture

[char0n]

This architecture should allow us to reuse part of other parsers and compose them.

[char0n]

Linked PR contains major refactor of current OpenAPI3 JSON adapter. Goal of refactor was to allow easy extensibility of the adapter. Before the refactor there were two parallel concepts: parsers and visitors. In this refactor parsers have been fully absorbed into visitors so that there is only one concept - visitors. Future optimization of unnecessary visitor traversal will be possible via special short-circuit version of traversal function.

Key aspect of this plugginable architecture is specification object which contains tree of visitors that maps to AST during traversal. Specification object can be easily manipulated. Any part of the tree can be replaced by custom implementation. Every visitor uses specification object to get it's child visitors instead of importing it using JavaScript modules.