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.
1) I'm not sure what the best heuristic of a filename is when you have dots in it. What should `todo.html.bak` map to? `todo.elm`? `todo.html.elm`? I know we have another solution in the works where you actually specify the file. That would solve this problem.

2) We want to know the relative file path of the main elm file.