Allow users to specify no display

This generalizes the IJulia detection, allowing users to turn off Winston's backends by specifying their `output_surface` to be `:none`.

Just like IJulia doesn't require a tk/gtk backend, specifying `:none` will prevent loading tk or gtk.  This is useful for saving plots directly to file without launching the GUI frameworks or when using [TerminalExtensions.jl](https://github.com/loladiro/TerminalExtensions.jl).
Merge pull request #154 from mbauman/no-display

Allow users to specify no display