Need to have the ability to add error bars to the bar chart

[sanjaypojo]

We need to add errorBarMinKey which defaults to errMin and errorBarMaxKey which defaults to errMax as props so that users can pass errors in each row of their data. We can also do a PropType check where we check that row[errorBarMinKey] < row[weightKey] < row[errorBarMaxKey], because we'd get strange error bars otherwise.