Add Network.Wai.UrlMap

This module gives you a way to mount applications under sub-URIs. For example:

```
bugsApp, helpdeskApp, apiV1, apiV2, mainApp :: Application

myApp :: Application
myApp = mapUrls $
      mount "bugs"     bugsApp
  <|> mount "helpdesk" helpdeskApp
  <|> mount "api"
          (   mount "v1" apiV1
          <|> mount "v2" apiV2
          )
  <|> mountRoot mainApp
```

Then, `bugsApp` will be accessible under `/bugs`, and, for example, a request to `/bugs/show/35` will be seen by the bugs application as just `/show/35`.