Regularization

[prcastro]

We should make regularization available.

Would it be feasible to make it extendable? The user could define a function of the weights to be minimized alongside with the error, and we use this function (its derivative) to update the weights? Or should we hardcode some of them (L1, L2 and Dropout, for instance) into Nervoso?

Useful link:
http://neuralnetworksanddeeplearning.com/chap3.html#regularization