this.extend() should copy extendTraces api

[bcliang]

Alternative to #6

extendData property should expect to receive an object {update, indices, maxPoints}

[bcliang]

... going to close this and stick with #6.

If need to stick with explicit extendTraces api, should be done as a modification to dcc.Graph directly. I have a PR there plotly/dash-core-components#461