Support static labels (without fading) in Pie charts

[g1eng]

Problem

setLabel method in Pie layer cannot parse Layer.label.
So any static label, which is shown by default, cannot be rendered.

In principle, Pie overrides setLabel and it should support the same behavior with other layers.
FigConfig API on display control on static label for Pie charts, should be supported.

Proposal

Implement internal process which parses FigConfig.label in Pie constructor.
Relative field name and its role can be following one:

{
  label: {
    visible: Boolean,  // label visibility control
    rotate: Number,  // label rotation control
    fade: Boolean   // whether its labels is static or not
  }
}