Add bar charts

[kdorr]

Bar charts aren't supported yet in mpl-altair.

Implementation Notes

• Marks should be handled in _marks.py
• Plotting different data types in a bar plot gives different types of bar charts in Altair (vertical, horizontal, skinny bars, fat bars).
• Plotting a categorical y against a categorical x gives a heatmap-like chart in Altair.
• Adding a color encoding does different things depending on the data type. Sometimes the chart gets stacked, sometimes it doesn't.

For more information

• Altair user_guide