when using dark theme and background_gradient - font should go to black?
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd
import numpy as np

df = pd.DataFrame(np.random.randint(0,5,size=(5, 4)), columns=list('ABCD'))

df.style.background_gradient(cmap='viridis')

df.style.background_gradient(cmap='Wistia')
```
#### Problem description
Currently when switching to jupyter lab's dark theme the pandas table output automatically switches to white text which is quite nice.
When applying a style with a color gradient, I have seen more often than not that a black font for the text would be nice.  

I don't think there is any right answer here - because at times even with a black font with some of the color maps it becomes un-readable for both light/dark themes.

Here are some images of color maps being applied side-by-side with the light/dark theme in jupyter lab:

![image](https://user-images.githubusercontent.com/1798497/47801495-b3be4900-dd04-11e8-8d81-c93e26f1992e.png)



Perhaps it would be nice to add a parameter that would enable the choosing of the font color, so one could specify white or black.   

I see from looking here:
https://pandas.pydata.org/pandas-docs/stable/style.html

That I can do this:
```python
df.style.set_properties(**{'color': 'black'}).background_gradient(cmap='Wistia')
```
which produces this:
![image](https://user-images.githubusercontent.com/1798497/47802163-19f79b80-dd06-11e8-84a5-fdf9b6e84fa9.png)


So perhaps
```python
.set_properties(**{'color': 'black'})
```
might be the best option here, but just thinking of doing quick analysis it might be more intuitive/nice to have:
```python
df.style.background_gradient(cmap='Wistia', font_color='black')
```