New feature: add color argument to tree export_graphviz (at least for classification)

[bertrandhaut]

The export_graphviz when applied on a classification tree can color the node according to the assigned class.

The color code used for that class is automatically built using the _color_brew function.

It would be nice to let the user gives these color as argument to the export_graphviz function.

A typical use case is when the user is already creating other graphs with color assigned to the class. It will be nice to have the same "color code convention" on all graphs.

[jbeanz]

/take