Fix #71 Hard dependency to MVC setup

[JanMattner]

Offers decoupling of the dependency to MVC setup in IApplicationBuilderExtensions. Default behavior is still to use MVC.

[jaredcnance]

at this point I would question whether or not it should be the responsibility of this library to even call .UseMvc()

[JanMattner]

I think so too. However, that would be a breaking change.

[jaredcnance]

Thanks for the PR. I'm fine with the proposed solution. I'd just like to see if there is maybe a better one.

Since the library won't even work without the MVC middleware, what do you think of just adding an overload with a Action<IRouteBuilder> configureRoutes parameter which we can pass into the UseMvc method?

[JanMattner]

Yes, I thought about that too. Currently, that is the only available overload for UseMvc, so that would be fine. As soon as there is another overload, you'd need to make this available as well...

Furthermore, I think it matters in which order the Use-Calls occur, so to be completely flexible, another Action<IApplicationBuilder> would be necessary that is called between the app.UseMiddleware<RequestMiddleware>() and app.UseMvc() statements.

[jaredcnance]

@JanMattner all good points. I can't think of any circumstances where someone would need to apply middleware in between the two, but that doesn't mean there isn't a use case. That would result in a method signature like:

IApplicationBuilder UseJsonApi(this IApplicationBuilder app, 
  Action<IRouteBuilder> configureRoutes, 
  Action<IApplicationBuilder> intermediateAppBuilderCalls);

which I can't say I'm a huge fan of. I don't think we should further couple it to the MvcApplicationBuilder api. Ideally I'd like to see this deprecated in the future. But, I think this PR is a good start.