Add MinDocCount and ExtendedBounds to Histograms

Unfortunately this is a bit hacky to fit in with the expected DSL. I'd be keen to see if there's another way.

```
numHisto := Aggregate("p").Histogram("price", 50).MinDocCount(float64(0)).ExtendedBounds(nil, float64(0))

time2013, _ := time.Parse(time.RFC3339, "2013-01-01T00:00:00+00:00")
time2014, _ := time.Parse(time.RFC3339, "2014-01-01T00:00:00+00:00")
dateHisto := Aggregate(
	"articles_over_time",
).DateHistogram(
	"date",
	"month",
).MinDocCount(
	float64(0),
).ExtendedBounds(
	time2013,
	time2014,
)
```