Allowing for templated urls in iFrame (#460)

* Allowing for templated urls in iFrame

This can allow for passing {{ width }} and {{ height }} as dynamic
attributes in the iFrame's URL.

The new method Slice.render_template method could do more eventually
exposing more variables to be used in dynamic strings.

* Passing function references

* js linting

caravel/assets/javascripts/modules/caravel.js

@@ -1,6 +1,7 @@
 var $ = require('jquery');
 var jQuery = $;
 var d3 = require('d3');
+var Mustache = require('mustache');
 
 // vis sources
 var sourceMap = {
@@ -215,6 +216,13 @@ var px = (function () {
       getWidgetHeader: function () {
         return this.container.parents("li.widget").find(".chart-header");
       },
+      render_template: function (s) {
+        var context = {
+          width: this.width,
+          height: this.height
+        };
+        return Mustache.render(s, context);
+      },
       jsonEndpoint: function () {
         var parser = document.createElement('a');
         parser.href = data.json_endpoint;