Add chart.api.rotate() method

I am trying to rotate an axis after `c3.generate({...})` has already been called and am running into issues with the axes. They are not rotating properly.

I'm currently using `chart.internal.config.axis_rotated=true` to attempt this but the axes are not displayed correctly (see below)

An example of a working chart that is not rotated:

The same chart after applying `chart.internal.config.axis_rotated = true`
Notice that the x axis (supposed to display a day) is squashed and not on the left side. The y1 and y2 axes are not displaying correctly as well.

Is there a way to fix this? Thanks!