Allow specifying PathView class

[AngellusMortis]

Is your feature request related to a problem? Please describe.

No.

Describe the solution you'd like

There are cases where it would be nice to be able to override the PathView class. So it would be nice if you could pass the class to use into the Router without needing to override it as well.

My specific use case is for New Relic monitoring (which I would probably consider out of scope of this library). New Relic is not detecting the name of the API operation correctly so I need to add a set_transaction_name call inside of PathView to fix it.

[bqumsiyeh]

I found a workaround for getting it to display the function name properly in NewRelic, see #428 (comment)