Barplot add space option
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] Adds `space` option to bar charts, which adds space between columns. This is nice especially if the user is making a grouped bar chart.

Example figure
![example](https://user-images.githubusercontent.com/7705285/47348034-3ddd3080-d6b9-11e8-8380-3ada4f8b6cbe.png)
:
