skip-prone? predicate option for wrap-exceptions

I use various http clients to test apis (postman, httpie, emacs restclient) but they don't do well with prone's output. This allows you to pass in some header patterns that should be excluded from prone without having to deactivate the middleware. It also allows the handy pattern of using `(debug)` and viewing prone's html in the browser while using an http client to view the api response.

```clj
(-> app
    (prone/wrap-exceptions {:excluded-headers {"user-agent" #"HTTPie/.*"}}))

(-> app
    (prone/wrap-exceptions {:excluded-headers ["user-agent" #"HTTPie/.*"
                                               "user-agent" "Emacs24"]}))
```