Add placeholder img helper

[tylersticka]

Almost all of our prototyping projects benefit from a placeholder image solution, but most have caveats:

• Placeholder images we save to project asset folders have to be created and managed manually.
• Services like placehold.it or PlaceIMG require external requests, which can hang if connections are slow.
• Client-side solutions like Holder.js require JavaScript, which can be a little tricky to integrate if the project includes both production and documentation-related JavaScript toolkits.

I prototyped a solution that uses a Handlebars helper to generate an SVG based on the provided attributes, encoding it as a data-uri for use in templates. You can see the working prototype here: http://codepen.io/tylersticka/pen/7119ac1f7900118fae7cd5eab2f6e2fa/left?editors=1010

The prototype is tested and working in the latest versions of Safari (desktop + iOS), Chrome and Firefox, as well as IE 10 and up. Tasks remaining:

• Rewrite as a module within this repo
• Write tests for it