Debugging custom HTML (e.g., todomvc)

### Features
* Debug any file by appending `?debug` to the URL. If you have a custom HTML file with your own styles, you may now debug it. Debugging `todo.html` just got easier.
* HTML files show up as debuggable. 
![screen shot 2014-07-28 at 6 03 22 pm](https://cloud.githubusercontent.com/assets/1889997/3723161/bee4bfc6-1670-11e4-8dc1-b8397d207d3c.png)

#### Notes
There is still some complexity in debugging a custom file. In order to hotswap with your own HTML file a few things must hold true:
* Your HTML file name must reflect its relevant elm file (e.g., `todo.html` and `todo.elm`). Without this requirement, it would be very difficult to determine which elm file should be compiled and sent to the debugger on hotswap. Imagine you're working on `index.html` and it uses `todo.elm` as the main elm file. Suppose that the main file depends on `item.elm`. When you edit `item.elm`, would we call `elm --make --only-js` on ` item.elm` or `todo.elm`? By forcing the html file's name to match the main elm file, we evade this problem.