Feature request: Cross-entropy error function

[tvedebrink]

In nn.ts it seems as only quadratic loss is supported in the playground? The definition of the Error class seems to indicate that one could implement the cross-entropy error function rather easy. With target being $t$ and $o$ for output The cross-entropy is given as
$f(o,t)=t\log (o)+(1-t)\log (1-o),$
where the derivative with respect to $o$ is
$f^{\prime }(o,t)=\frac{\partial }{\partial o}t\log (o)+(1-t)\log (1-o)=\frac{t}{o}-\frac{1-t}{1-o}=\frac{t-o}{o(1-o)}$
In this case, is my suggestion below correct?

public static CROSSENTROPY: ErrorFunction = {
    error: (output: number, target: number) =>
               target*log(output) + (1-target)*log(1-output),
    der: (output: number, target: number) => (target - output)/(output*(1-output))
  };

In the playground.ts script I would then change all references to SQUARE to CROSSENTROPY or make it possible to select the error function by drop-down menu?