Add Rails-style lambda constraints

This would allow a Rails-style lambda constraint so you could say something like:

```coffeescript
match '', 'users#dashboard', constraints: -> isLoggedIn()
match '', 'pages#startPage'
```

It needs tests before being pulled, but I wanted to see if there was interest in such a thing. I know you can basically accomplish the same thing in the controller, but I just like this method better.