Support passing initial code in query param 

This adds support to pass code via a query param. 
e.g. 

http://scalatutorials.com/?code=var%20x%20%3D%201%20%2B%202%0Aprintln(x)%0Ax%20%3D%203%20*%204%0Aprintln(x) 

opens the editor with this:

```scala
var x = 1 + 2
println(x)
x = 3 * 4
println(x)
```

There is a limit to the code length as it's passed via GET params, but it should support most use cases I think...