Make option creation and provider creation public and more generic

[cervengoc]

With this PR it's possible to use the generator outside of the WebAPI pipeline using the same configuration without needing to manually set up the generator and the options.

This is needed to ensure that for example a unit test is using the exact same configuraiton as the Web API at runtime.

An example pseudo-code from a possible unit test

var config = new SwaggerDocsConfig();
var httpConfig = new HttpConfiguration();
WebApiStartup.ConfigureApi(httpConfig); // this is called by the API pipeline too
WebApiStartup.ConfigureSwagger(config);
var generator = config.GetSwaggerProvider(httpConfig);
var document = generator.Generate....