ENH: Allow elementwise coloring in background_gradient with axis=None
At present the axis argument to `background_gradient` only accepts 0 or 1 so that the resulting "heatmap" can only be applied columnwise or rowwise rather than to the whole table.
```
Signature: df.style.background_gradient(cmap='PuBu', low=0, high=0, axis=0, subset=None)

Parameters
----------
cmap: str or colormap
    matplotlib colormap
low, high: float
    compress the range by these values.
axis: int or str
    1 or 'columns' for colunwise, 0 or 'index' for rowwise
subset: IndexSlice
    a valid slice for ``data`` to limit the style application to
```
I propose that `axis=None` be allowed to mean applying a color gradient elementwise.