Tooltip formatter neglects all other options

[hazirmagron]

Is your feature request related to a problem? Please describe.
When the Tooltip formatter is used the other options, such as valueFormatter, textStyle and others are neglected.

This is for the most part fine since value formatting can be done inside the formatter directly. I have yet to find a way to apply styling. Not being able to apply styling is particularly problematic as the one applied when formatter is used shows the tooltip text in a very small font.

Edit:
I might add that the behavior is inconsistent. E.g. tooltip.textStyle.color does still apply when formatter is used, fontSize does not apply.

[hazirmagron]

Got any suggestions? @zhiqingchen

[zhiqingchen]

Similar issues:

#15