heatmap performance

I suspect this isn't heatmap-specific, but that's the context in which I'm seeing it pop up.

In a Juno Session, running

```julia
using PlotlyJS
X = rand(1025, 5000)'
plot(heatmap(z=X))
```

I use this sort of thing for plotting audio spectrograms.

On my machine the `plot` line takes about 12s before it looks like the Julia function returns (I see a result in Atom and the spinner animation stops), and then 9s more before the heatmap is displayed.

Timings also seem about the same when running from the REPL.

[here](https://jsfiddle.net/xfj1yxyu/) is a JSFiddle with a pure-javascript version.

Clicking `run` displays the plot in about 2.5-3s, which also isn't great, but it seems like the Javascript side isn't the main bottleneck.

I'm open to the idea that this is an unreasonable use-case for PlotlyJS, but I just like it so much for all my other plotting needs I want to use it for everything. :)