Changes:

• Add trail mark type for variable-width lines.
• Add aspect property for image mark aspect ratio control.
• Add support for signals and null fields in comparator definitions.
• Add mark property config options based on role annotation.
• Add elBind parameter to View.initialize, to add bindings to a custom DOM element.
• Update custom symbol paths to scale size by radius, not diameter.
• Update signal bindings to populate an internal div element by default, rather than the top-level view container.
• Update vega-force to use current d3-force version.
• Fix protocol check bug that was breaking baseURL application in vega-loader.
• Fix canvas renderer sub-pixel damage region bug.
• Fix legend format specifier bug.
• Fix View insert/remove data API bug.
• Fix equality checks for DOM-binding input element values.