UI Configuration Improvments

This pull request does the following:
1. Defaults the UI to point at the configured basePath and endpoint not the petstore api.
2. Allows you to disable the ui and/or directory listing functionality.

Looks like that along the way the default ui no longer pointed to the local api and only if you didn't modify the endpoint setting.  This pull request corrects this server side so that it can have access to the endpoint configuration.  Handlebars was already listed as a dependency so I used that.

Also in some cases it might not be desirable to have the ui hosted on the api server and or provide directory listings of the swaggerui.

I thought that others might find them useful; however, feel free to reject this request.