ENH: I wish add a parameter to adjust bar function's height in Styler
#### Is your feature request related to a problem?

I have a 'Styler' object, I wish add a parameter that can adjust bar's spacing to the [bar function](https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.io.formats.style.Styler.bar.html).

eg:
now the bar example like bottom:
![image](https://user-images.githubusercontent.com/36298417/93431626-fc7e4b80-f8f6-11ea-8ebe-453b6ce4d3b9.png)

but I wish like bottom:
![image](https://user-images.githubusercontent.com/36298417/93431767-2fc0da80-f8f7-11ea-8284-61b7fe02df1b.png)

Thx!!!

