Return X locations of bars

[scottclowe]

If the user allows superbar to automatically determine the x-location of the bars (the expected typical usage), they won't know where the bars are actually located. This can be a problem if they want to use customised x-ticks or change the x-limits, for instance.

We should return the locations of the bars for the user's convenience.

However, this require an API change, so should be released in v2.0.