Allow each portfolio to have different rebalancing parameters

[dppalomar]

In the current package version, the rebalancing parameters are common for all the list of portfolio functions to be backtested. However, it may be interesting to be able to specify different parameters for each strategy function. One example of use would be to try the same strategy but for different rebalancing frequencies.

This feature will be included, hopefully in a nice way that doesn't destroy the simplicity of the interface.