Add attribute `partial-graft`

Closes https://github.com/Shopify/turbograft/issues/6 (more or less)

This PR adds a helper attribute and JS handler to facilitate GET-style partial refresh.  It can be used on any DOM element

It looks like this:
`<button partial-graft href="<%= page_path(@id) %>" refresh="section-a section-b">Partial Graft Helper: A and B</button>`

Clicking this will GET and replace `section-a` and `section-b` refresh areas

cc @nsimmons @tylermercier for review
Maybe move the function out so this comma isn't hiding down here?

```javascript
doit = (ev) ->
  target = ev.target
  partialGraft = target.getAttribute("partial-graft")
  href = target.getAttribute("href")
  refresh = target.getAttribute("refresh")
  return unless partialGraft != null && href != null && refresh != null
  
  keys = refresh.trim().split(" ")
  
  Page.refresh
    url: href,
    onlyKeys: keys

document.addEventListener 'click', doit, true
```