Feature: Include router option that prevents over writing routes

I discovered a scenario where you can create two handlers that handle the same pattern but the second one "overwrites" the first one.

The expected behavior would be to panic. Because the pattern `GET /things` being attempted to be added twice

```go
baseRouter := chi.NewRouter()

baseRouter.Get("/things", func(w http.ResponseWriter, r *http.Request) {
w.Write([]byte("things"))
})

things := chi.NewRouter()
things.Get("/", func(w http.ResponseWriter, r *http.Request) {
w.Write([]byte("other things"))
})

baseRouter.Mount("/things", things) //should panic
```

The example below I would not expect to panic because there are no overlapping URL patterns. The are `GET /things` and `GET /things/other`

```go
baseRouter := chi.NewRouter()

baseRouter.Get("/things", func(w http.ResponseWriter, r *http.Request) {
w.Write([]byte("things"))
})

things := chi.NewRouter()
things.Get("/other", func(w http.ResponseWriter, r *http.Request) {
w.Write([]byte("other things"))
})

baseRouter.Mount("/things", things) //should not panic
```

For a `GET /things` the response will be `other things`