add Plots extension

[hhaensel]

Referring to #13 I propose an additional extension that adds a render function for Plots. That way, we could also allow for

@app Example begin
    @out plot = Plots.plot(20:-1:1)
end

ui() = plotly(:plot)

[hhaensel]

Currently I moved the required Plots.plotly() to insure the Plotly backend is loaded to call to __init__().
Alternatively we could place it in the render function:

function Stipple.render(pl::Plots.Plot)
    if isdefined(Plots, :plotlybase_syncplot)
        current_backend = Plots.backend()
        Plots.plotly() === current_backend || Plots.backend(current_backend)
    end

    pl = Plots.plotlybase_syncplot(pl)
    delete!(pl.layout.fields, :height)
    delete!(pl.layout.fields, :width)
    pl.layout.fields[:margin] = Dict(:l => 50, :b => 50, :r => 50, :t => 60)

    return pl
end

@essenciary what do you think?