Make it easier to extend/subclass default router

[vytas7]

We are getting feature requests and questions from the community to be able to route based on different properties of req than just path, see, e.g., #709.

We probably don't want to bloat the default router too much, OTOH, after #967, custom routers can go wild with the passed req object. However, at the time of this writing, it is a bit cumbersome to implement a new custom router -- although the protocol itself is straightforward and easy to understand, implementing a completely new routing strategy from scratch is quite an involved task, and it is unclear how to extend the default "compiled" router beyond simple wrappers around, e.g, add_route().

Things that already are easy to reuse:

• map_http_methods() -- factored out into a separate helper that can be used with any router.
• set_default_responders() -- factored out into a separate helper that can be used with any router.

Things that are opaque/private/undocumented/hard to reuse:

• Extracting parameters from a URI template. Controlling how parameters are passed to a method. Unavailable in falcon.inspect either.
• Parsing converter spec from a URI template, controlling how converters are called. Unavailable in falcon.inspect either.
• Reusing/extending the compile machinery, all methods/locks are private except the compile flag itself.
• Extended code generation in any way.