WIP: mplexporter / pyplot interface

Very WIP. Currently just translates + updates the code @rossant had working.

The long todo list:

* [ ] Make text work.
* [ ] Complete text display options.
* [ ] Make `LineVisual` and `MarkerVisual` proper visuals in `scene/visuals`.
* [ ] Complete support for line/marker properties.
* [ ] Change to use `SceneCanvas`. This should facilitate:
* [ ] Deal with different plotting coordinate systems.
* [ ] Deal with multiple axes. 

Things that would be nice, but not 100% necessary to have things working (and probably difficult):

* [ ] Make collections work (efficiency).