Allow for different rebalancing schedules

[dppalomar]

In the current package version, one can specify every how many days the portfolio needs to be rebalanced/reoptimized (with the arguments optimize_every and rebalance_every. However, one may want to specify a different rebalancing schedule, like every Monday or every last day of the month.

This feature will be added, hopefully in a way that the interface is still clean and simple.