support passing code in query parameter

e.g. this request:
localhost:8080/?code=var%20x%20%3D%201%20%2B%202%0Aprintln(x)%0Ax%20%3D%203%20*%204%0Aprintln(x)

will result in the editor opening with:

var x = 1 + 2
println(x)
x = 3 * 4
println(x)