Fix for #1277 - Add Options/Startup API for WebAPI shim

Adds an options class, as well as a default options setup that will
configure the default set of formatters.

Currently most of what options needs to do is a placeholder, but it later
do things like add ApplicationModelConventions, filters, formatters, model
binders, etc. Those will be added in follow up items.
We have a bug to do this for `MvcOptions`, but since this is a new type, you might want to follow the pattern SignalR does with the `Add*` - https://github.com/aspnet/SignalR-Server/blob/dev/src/Microsoft.AspNet.SignalR.Server/SignalRServiceCollectionExtensions.cs#L39.

Basically return a `WebApiServiceCollection` that has a method `ConfigureWebApi` which is an `Action<WebAPIOptions>`. So your code would look like:

```C#
services.AddWebApiConventions()
             .ConfigureWebAPI(webAPI =>
             {
             });
```