Mocks here: https://www.figma.com/proto/icviajRVLhC69Ud0OW7dKe/dev-tools?node-id=162%3A2

This involves two things:

1. Providing a CSS file to re-skin the werkzeug debugger
2. Implementing the dash dev tools UI in the dash-renderer library.

@rmarren1 has written the skeleton UI for dash dev tools #64. @valentijnnieman will work off that branch to implement the UI on top of it.