adds ability to reset params after a filter runs

When a filter is run _after_ the router completes, it's input params,
such as `":splat"` will have been overwritten by the filter's router pass.
This commit adds the ability to tell the router to revert to the previous input
params after running a filter.

filter.go

@@ -27,6 +27,7 @@ type FilterRouter struct {
 	tree           *Tree
 	pattern        string
 	returnOnOutput bool
+	resetParams    bool
 }
 
 // ValidRouter checks if the current request is matched by this filter.