First try at implementing dashed lines.

**Not working**

I'd really like to have dashed lines (like 3. in #201) now that there's dcjones/Compose.jl@a781354. This is my try at it, but I failed miserably. When reaching `render(geom::LineGeometry ...`, `aes.dash` is always `nothing` when I call `plot(sin, 0, 10, dash=[5mm, 5mm])`.

Really, this PR is just to get the ball rolling; feel free to ignore it completely and implement this feature the right way right away :smile: