export_graphviz support for regressor tree

Is there any particular reason why [tree.export_graphviz](https://github.com/scikit-learn/scikit-learn/blob/ab93d65/sklearn/tree/export.py#L70) is not supported for a decision tree regressor?

From the docstring:

""" ....
Parameters
    ----------
    decision_tree : decision tree classifier
        The decision tree to be exported to **GraphViz**
... """

