Add class to elm debugger

[neslinesli93]

This PR adds the class __elm__internal__debugger__ to the widget that lives to the bottom-right of the screen and is used to open the debugger.

This is useful for possible overrides that users may want to do (move the debugger to the right, lift if up a little, etc).

I'm open to discussion, but this would be very useful for us, since we've a toast notification system that doesn't work well in development since the huge z-index of the said widget doesn't allow for any exact override. A class would be a huge helper, and hopefully has a minimal impact

[rupertlssmith]

I wonder where you would document this, so that people know this CSS class is available for modifying the debugger?

[rupertlssmith]

Would it be better to have an id instead of a class? Only going to be one debugger.

[rupertlssmith]

Would be interesting in including this PR in elm-janitor: https://github.com/elm-janitor/manifesto

There are conflicts with the 1.0.2 release tag. Would you rebase this PR onto 1.0.2 so it can be merged?