Merge 1.3.0-preview changes into master branch for release

[darrelmiller]

Enhancements

• Updated framework dependencies to just .net standard 2.0.
• Roundtrip the parameter name of a body for OpenAPI v2 documents
• Improved support for multi-line strings. Add literal style support for OpenApiYamlWriter #481
• Remote references are supported (currently limited to absolute paths)
• Added validation for path parameters defined that are not in path. Feature/add validator for path parameters not present in path #524

Bug fixes:

• OpenAPI Walker missed a variety of elements OpenApiWalker does not Walk the schema in AdditionalProperties #484 Walker does not visit OpenApiEncoding.Headers #405
• Don't fail when using $ref in path Item $ref is not a valid property path item #288
• Specification extensions for external docs now supported Specification Extensions not supported for externalDocumentationObject #513
• Fix for default value of Explode Query Parameters Serialization - Explode don't respect the default value. #479
• Removed validation of examples by default format:int64 parameter throws data mismatch error #407