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 is 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!