Attribute Routing: Design a scheme for token replacement in templates

[rynowak]

We've had a few requests for the ability to write code like this:

[Route("api/{controller}/{action}/id?}")]
class BaseController : Controller
{
...
}

class BlogController : BaseController
{
...
}

Ultimately what the user wants here is for the {controller} token to be replaced with the controller name and the {action} token to be replaced with the action name(s).

We should consider whether or not this is truly a parameter (with parameter syntax) or is just token replacement (with different syntax).

Includes discussion about what the exact meaning of [Route("api/{controller}/{action}/id?}")] is. Is this token replacement? If we support token replacement, are these specific parameters still supported.

[rynowak]

Need to update my PR after receiving feedback. 1 day or less remaining