Add weighting parameters to edit distances

[OlivierBinette]

Edit-based distances can have their different operations (transposition, substitution, deletion, or insertion) be weighted to allow more flexibility. It would be nice to provide this functionality to the package.

We need to keep in mind that users might want to be able to learn these weights from data. So tuning these parameters should bring as little computational overhead as possible.

It would be best to wait working on this issue until we have a working case study for which we want to implement string distance learning.