add tickFormat as prop to BarChart

[mskeving]

Bug

Currently there is no way to pass in a custom tickFormat function to BarChart unless you include it in your x or y scale. However, Axis is set up to check if a tickFormat prop is defined independent of scale.

Fix

Allow BarChart to receive a tickFormat prop. This allows you to customize your tick labels without having to worry about figuring out what the rest of xScale entails.

Tested

I tested this with a graph I've been working on. I only wanted to show every 4th tick label, which I can now do by passing in my own function as a tickFormat prop to BarChart. If no tickFormat prop is defined, business continues as usual.

[codesuki]

Very nice pullreq, easy to understand. Thank you!