Implement new plotting features

[pviscone]

Implement new arguments to pass to the Hist classes like

• Callback function to modify the hist.Hist object before plotting (both hist and its ratios)
• Callback function to modify the matplotlib ax object before plotting (both hist and its ratios)
• A ratio flag to enable/disable ratio
• An argument to override the default --ratio argument setted in the cli to impose which hists to use to compute the ratio
• Flags to include/exclude processes. It can be something like the one defined for targets, e.g. onMC, onData, onDataDriven, onSignal, onBackground, processPattern
• ! Move all plotting arguments in the cli to the defaults

Also separate consistently the main plot kwargs, the ratio kwargs, the stack plot kwars, the total stack plot kwargs, the data plot kwargs, the legend kwargs, the subfigure kwargs, and the axis options.
For each one of this create a callback function that modify the hist, and another one that modify the ax. The callback function must take other arguments, so use partial from functools.

Callback functions must be passable also as defults