Calling Other Contract with interface

[t4sk]

Relevant PR https://github.com/solidity-by-example/solidity-by-example.github.io/pull/247

[t4sk]

I think it's easier to understand if the interface is defined inside the current example.

contract Callee {
 // code
}

// Define interface if code of callee is not available or not needed
interface ICallee {
}

contract Caller {
 // code
}