[v2.0] More options for the history graph of numerical habbits

[Stiralbios]

I think it would be nice to have an horizontal line representing the threshold to reach on the history graph.

Right now if you change the time period of the history graph to month (or a larger time period) it add the value of multiple days onto one bar. It would be nice to be able to select the aggregate function it will use when combining days (add, mean, max, min, etc...)