Make charting magic commands accept any python data type as arguments

[robertbindar]

Currently the charting magics (%bar, %pie, etc) accept as arguments that are going to be passed to DataFrame.plot only floats, ints, bools and strings.
Whilst this is kind of enough to generate simple, not very customized charts, we should try to make these magic commands accept any other python data type that DataFrame.plot accepts as well (e.g. tuples, lists, and so on) to offer users the maximum flexibility to customize their charts.
This needs a bit of investigation to see if the ipykernel machinery the kernel inherits can help us cast a string (argument of a magic command) to the proper Python data type.