support dom element as argument to Plotly.makeTemplate

[antoinerg]

As a first-time user of templates, I expected the following to just work:

var.gd = document.getElementById('graph');
Plotly.plot(gd,{...});2,3], y:[4,9,3], line: {color: 'red'}}
...
var layout = Plotly.makeTemplate(gd);

This PR enables calling makeTemplate with a DOM element.

[alexcjohnson]

Thanks @antoinerg - beautiful test. After the indentation fix, 💃 !