histogram: option to enable/disable zero-fill

For some graph types (particularly point style), it's useful to
explicitly avoid filling missing data points with zeros.  For example,
when plotting the min/max/mean value of a particular field, filling with
zeros might present the wrong picture of the data, since the value is
not actually zero but simply missing.

This is simply a more recent rebase of https://github.com/elasticsearch/kibana/pull/612 after a long hiatus in development.