Generate TOCs for specification docs

[scottydawg]

This change adds a gulp task for generating a variant of the markdown specification with a table of contents (using the DocToc library).

The swagger specification document is a bit unwieldy as a reference point because:

• It has no ToC to provide a high-level context of what is contained in the specification, and
• The styles used on http://swagger.io/specification/ visually deemphasize h4 elements; h5 elements are more pronounced. This makes the hierarchy difficult to navigate.

This change doesn't address styling issues on swagger.io, but does allow generation the specification document with a ToC.

[ePaul]

Could you rebase your Pull request against current master (or maybe even better, current OpenAPI.next)? There the specs are in the versions folder.

[webron]

We can consider this for 3.0, but we shouldn't make the changes to 1.2 and 2.0.

[webron]

Closing as we'll be introducing only must-have updates to the specs (typos or 'bug' fixes).