Implement Whoops feature to open referenced files in editor/IDE

This PR updates the `filps/whoops` dependency to `1.0.1`, which includes among other small fixes a feature to enable opening files referenced in the error page in the user's editor/IDE of choice. Required changes to templates & CSS were also ported.

![Whoops!](http://i.imgur.com/7YHsYZo.png)

Currently, the feature will not be visible, as no editor will be set by default. I've decided against making the choice to structure that myself. My idea was to add another option to `app/config/app.php` named `editor` or some such, but there's probably room to allow machine-specific editor choices, loading from environment variables, etc.

Setting an editor is simple:

1. Check the available editors in `PrettyPageHandler`
2. Use `PrettyPageHandler::setEditor( string $identifier )` to set an editor by identifier
3. Alternatively, pass a callable to `setEditor` that accepts a file path and a line number, and returns a string usable as an `href` property
4. Alternatively, add your own editor options with `PrettyPageHandler::addEditor(string $identifier, callable)`

Finally, @daylerees might want to have a look at my changes to the template and CSS, and tweak them to fit his choices - my solution is far from perfect :sweat_smile:

Additional info is available [in the whoops repo](https://github.com/filp/whoops#opening-referenced-files-with-your-favorite-editor-or-ide).

Cheers!