Allow precomputed statistics input for box traces

For example:

var trace = {
  type: 'box',
  q1: 1,
  median: 2,
  q3: 3
}

See http://community.plot.ly/t/box-plots-manually-supply-median-and-quartiles-performance-for-alrge-sample-sizes/2459

[ekalosak]

This is a vital feature for industrial readiness. If data needed to compute quantiles cannot fit in memory or are expensive to shuttle across a netowork, the aggregations must be computed near the data store (SQL: materialized views, Hadoop: Map-Reduce). If we need to compute quantiles near the data store, it would be convenient to pass those summary stats directly to the plotting routine.

[rsandler00]

Would also love this feature!